So much skullduggery has eaten into the soul of Russia that not even a whistleblower's brutal end may cause the merest whisper of alarm.

Yet over recent weeks something unexpected has happened. Finally emboldened by the brazen vote rigging in December's parliamentary election, thousands of people shed their fear and took to the streets, and, defying any threat of a Kremlin crackdown, chanted the once unsayable: "Russia without Vladimir Putin."

From the polished charm of Chiswick on a bend of the river Thames, Marina Litvinenko watched those outraged citizens in Moscow and St Petersburg with mounting admiration and hope. "I'm very proud of them," she says. "At last Russians – and not just young Russians but those of the age of my mother, who is 75 – realise that to have a vote is a precious possession." It is five years since Marina was widowed; five years since the world witnessed Alexander Litvinenko's murder in slow motion as the former KGB officer-turned-dissident was transformed from full health into a helpless spectre, poisoned by polonium-210, a rare radioactive isotope, virtually undetectable and for which there is no antidote.

Litvinenko, who in 2001 had defected to Britain with his family, was dying in a London hospital, the victim of an enduring KGB mindset which punishes "traitors" in ways that spread maximum fear. So, he knew his killers, but more than that, he accused Vladimir Putin, Russian president at the time, of being directly responsible. In a signed statement, given four days before he died, Litvinenko's rage against his old boss was undiminished. "You have shown yourself to be as barbaric and ruthless as your most hostile critics have claimed," he declared. "The howl of protest from around the world will reverberate, Mr Putin, in your ears for the rest of your life -"

Today in sparkling winter sunshine Marina and I walk around Chiswick where she now lives with her son, Anatoly, who was 13 when his father died. Her husband's pet name for her was Maryusa, and when, in halting English, she refers to him as Sasha, her voice is tender with emotion. The graceful calm which first distinguished her in the eyes of the media remains, and her face is less haunted by sorrow than when we met in those early, grievous months after the murder. Britain has given Marina a new circle of friends and campaigners as well as the strength of London's long-established Russian emigre community. A pivotal figure in this story is also Boris Berezovsky, the exiled oligarch whose colourful breach-of-trust battle against Chelsea football club owner Roman Abramovich reads like a film script in waiting; a quintessential tale of the bitter, paranoid world of secrecy and greed among the stupendously rich.

In Russia, and later London, everything about Litvinenko's activities – along with those of his friend, the investigative journalist Anna Politkovskaya – propelled him towards dangerous collision with the Kremlin. As an officer in the Federal Security Service – the "reformed" KGB – Litvinenko had discovered that some of the Soviet agency's most ruthless figures were again in the ascendant. He and Politkovskaya believed senior officers had fabricated terrorist attacks in Moscow, claiming Chechen involvement as a pretext for launching a second war in the southern Russian republic in 1999. That conflict ensured a landslide presidential victory for Putin, and again, like Politkovskaya, Litvinenko became conspicuously vocal about the former KGB chief's "dirty war".

But in 1999 he was also under orders to kill Berezovsky. Instead he warned Russia's first billionaire that he was on the Kremlin's hit list as a political plotter out to bring down Putin, to whom he was openly hostile.

In 2000, having survived several assassination attempts, Berezovsky fled to Britain where he lives surrounded by a phalanx of heavies said to be ex-members of the French Foreign Legion. One year later, in gratitude for his life, he helped Litvinenko to defect, and ever since the former mathematician who bankrolls some of Russia's opposition parties has supported the Litvinenko family through his civil liberties foundation.

In Moscow, Marina, who is 49, was a professional ballroom dancing teacher but she has not visited Russia since her husband's death. "I could go back in theory because I still have a Russian passport, but I can't risk it, I just can't." When she and Anatoly recently celebrated her father's 80th birthday, the family met on the neutral ground of Kiev in the Ukraine. "I had not seen my father for five years and that made me feel desperate, so it was wonderful to see him still in good health." Ideally she would love to resume her dance career but much of her time in Britain, Europe and the US is involved with human rights campaigns and promoting Death Of A Dissident, the book she wrote in collaboration with Litvinenko's friend, Alexander Goldfarb, in a major deal with Simon And Schuster. Goldfarb, a Russian scientist who defected to America in the 1970s and lives in New York, is the executive director of the Justice For Alexander Litvinenko Foundation, which aims to assist the murder investigation and Marina's legal costs to expose the truth.

In April the second stage towards a full autumn inquest will be heard in London, a reminder that the nature of the killing was made all the more shocking by its location: a crime carried out not in some dank, bandit alley of Moscow but in the bar of a fashionable Mayfair hotel. Once the pre-inquest formalities are complete, the final hearing is expected to follow a format similar to that of the 7/7 London bombings, which could mean the process lasts for months, running up costs in excess of £1 million. Might Berezovsky pick up the tab? "You know, he might feel this is the right thing to do, but maybe I should not be every time too dependent. Boris Berezovsky has been so helpful in ensuring Anatoly receives a good education at City Of London School For Boys, and I appreciate very much what he does, but he also has much responsibility to other people."

Marina knows the chance of bringing the true culprits before a British court remains less than remote. She knows, too, that the hearing will inevitably plunge her back into the trauma of her husband's suffering. But she has waited five years to reach this point, determined that the world should hear testimony that Litvinenko was "assassinated by order of the Russian state". When we first met, she spoke of dreading "emptiness", a familiar fear for the bereaved. "At the beginning I could say: 'This time last year Sasha and I did this, did that,'" she reflected. Today she says, "Now in one way I realise he has gone for ever, but when Anatoly and I talk of him it's also with happy memories, so it's as if he is all around."

But now as the single parent of an only child, Marina is aware she and her son, who is 18, must be sensitive about giving one another space. "For a long time Anatoly didn't talk much about what had happened," she recalls. "Today I would say he's back to his old self. But if a mother keeps a son very close to her, it can destroy his personal life so he cannot give love to anybody else." And her personal life? "Some time ago when I was criticising Anatoly, like a normal mother trying to make her child better, he told me: 'Mum, could you find someone to take your attention more from me and give it to this other person?'" How did she respond? "I asked: 'Would you be fine with that?' And he replied: 'Absolutely.' But if somebody did decide to be with me, this person would have to have a big, generous soul to accept and respect Sasha, and not be jealous. Still, I am not desperate. If such a thing happens, it happens. And I am not without love because I am surrounded by loving people, which helps very much."

Litvinenko was 43 when he died, and in their 13 years together he had always shielded Marina from the dangerous side of his life. But the attack in the Millennium Hotel on November 1, 2006 changed all that. Suddenly she was thrust against the end game of clandestine treacheries and intrigue. Except that in the matter of justice there has been no end game: Andrei Lugavoi – identified by Scotland Yard as the chief suspect – is expected to give testimony at the inquest by video link, secure in the Kremlin's insistence that extradition will never be considered because it contravenes Russia's constitution. In such circumstances is there much point in pursuing the case? "This isn't just about Sasha," says Marina. "It's about a radioactive terrorist attack, and everybody needs to know the truth of how and why the polonium was obtained. You can't just steal it and hide it away for five years – it has to be freshly made. Someone had to give the polonium to Lugovoi. Someone had to plan this operation."

Yet in the bleakest moments the pain of loss must seem beyond all healing. Marina disagrees, believing the present stirrings for accountability in Russia might eventually benefit her cause. Discontent has been growing since September when Putin, as prime minister, announced he planned to swap roles with Dmitry Medvedev by running in March's presidential election, a poll nobody expects him to lose. But the arrogance of that September statement, with its prospect of "perpetual Putin", riled many because it confirmed what had long been suspected: Medvedev was keeping the presidential seat warm for the man who has never stopped pulling Russia's strings since 2000. Significantly, in the December parliamentary poll Putin's United Russia party lost 77 seats, returning to power in the Duma, the lower house, with a greatly reduced majority of 238 seats out of 450, despite evidence of mass fraud.

"That pronouncement was a slap in the face of Russians who more or less said: 'You know what? It doesn't matter to us what you decide. It is more important what we decide.' So, they took to the streets, and I was proud of them because they didn't go out to make aggression. Their protest was peaceful, orderly, intelligent."

Was it Russia's Tahir moment? As recently as five years ago tanks would have crushed such demonstrations. Now, as in the Middle East, social media alerted the world to what was happening in real time, and not even Putin could risk images comparable to the brutality of Bashar al-Assad's regime in Syria. Yet like many Moscow watchers, Marina cautions against equating winter's protests to the Arab Spring. Instead she sees them as a first step in the electorate's campaign against gangsters in high places, a demand for "normality" which in time might lead people to investigate what happened to Alexander Litvinenko and all the other activists silenced for speaking the truth.

But the distance to go can be measured by the political reality of Russia today. Putin's party ratings may be down amid plunging living standards and rampant corruption. His macho-man photo ops may be greeted with derision, but still no other politician commands a personal approval rating of around 60%.

In Britain Litvinenko had continued his pursuit of evidence against Putin. In 2006 he and Marina gained British citizenship. Six days earlier, on October 7, Politkovskaya had been gunned down outside her Moscow apartment, and less than one month later Litvinenko was dying, the result of taking tea with two Russian "businessmen" in a smart London hotel. On that November afternoon he had arranged to meet Lugovoi and Dmitry Kovtun in the Pine Bar of the Millennium Hotel, and while his attention was momentarily distracted, Lugovoi allegedly slipped the deadly isotope into Litvinenko's green tea.

At home that evening he was violently ill, leading the couple to suspect E coli poisoning. But at University College Hospital, London, as his condition worsened, it seemed as if everyone, including some of the world's top medical specialists, had been drawn into a horrific Le Carre thriller. Alexander Litvinenko was dying from radioactive terrorism on British soil.

During the first pre-inquest last year, Marina spoke of how grief was exacerbated by the fact Lugovoi may never face justice despite the strong evidence against him. On that fateful British visit he had left a polonium trail across central London, in his hotel bedroom and also on the aircraft in which he returned home. Back in Russia, he became a right-wing nationalist with a seat in the Duma. As Scotland Yard's investigations intensified and attempts to question him were refused, Lugovoi accused MI6 of sanctioning the murder, then touted a more bizarre accusation: the death was suicide, a result of Litvinenko's guilt at having betrayed the FSB's code of silence. The old Soviet cold war tricks were back in play.

For its part, the Foreign Office is said to regard the death as a "criminal case" rather than an assassination or an act of terrorism, which suggests mending fractured diplomatic relations and trade opportunities with Russia is the priority. But in a telephone conversation, Foreign Secretary William Hague has given Marina a personal assurance that the case will not be dropped under any circumstances. And recently Lord Macdonald, director of public prosecutions at the time Litvinenko was killed, described the event as having "all the hallmarks of a state-directed execution - the deliberate infliction in full world view of a lingering radioactive death upon a man who the Russians knew was under the protection of the British state".

Did those who planned the attack believe polonium-laced tea would be a cleaner weapon than a gun? "To use a gun in London would not have been so easy because of too many CCTV cameras," says Marina. But for her the grotesque theatricality of that heinous act remains the worst reminder: in Putin's Russia truth and justice are easily trampled on by vengeance. "They must have thought Sasha wouldn't survive so long, but he lasted 23 days." Only hours before he died, the doctors finally identified polonium in his urine. "Without that discovery," says Marina, "the cause of death would have been unknown." A mystery, the perfect kill. n