Masha Gessen's visceral and alarming biography of the key figure in contemporary Russian – and possibly world – politics suggests, credibly, that Vladimir Putin is defined above all by his love for the old Soviet Union and in particular its intelligence agency, the KGB.

Putin not only served in the KGB. He adored it.

And so when Putin at last accrued real power, when he was given the chance to run the finances of Russia's second city – and his home city – St Petersburg, he based his carefully constructed power system on what he had learnt in the KGB. He exercised total control; any dissent was ruthlessly crushed.

But there was one important difference and, in describing it, Gassen shows her considerable psychological acuity. This new system would never, ever let Putin down, as the old USSR had. Central to Gessen's view of Putin is her theory that he took the collapse of the old USSR and the KGB to heart, as a bitter personal betrayal. So, in his home town, Putin set to work, despising "wishy washy democrats" as he accumulated power.

Gessen knows a lot about St Petersburg, formerly Leningrad, and one of the many virtues of her tough, angry biography is that it is not in any way Moscow-centric. She herself left Soviet Russia for the US when an adolescent, but returned in time to report on the collapse of the USSR. She writes with raw authority about Putin's rise.

If the child is father of the man, then Putin must be very hard and very nasty. Gessen's superb early chapters show that the key character-forming environment in Putin's youth was "the courtyard", the squalid well of the grimy apartment building in which he lived with his elderly parents. This filthy, litter-strewn place was a bleak playground for thugs and drunks. Putin, despite being physically slight, could hold his own with these brutish outcasts. The courtyard was a testing ground, where he proved himself.

In later life, he was quite candid, even boastful, about his nastiness in childhood and early adolescence, admitting that he was "a hooligan, a real thug". He had a ferocious temper and he refused to let anyone insult or humiliate him. He was determined to learn how to defend himself physically in any situation. He trained as boxer, but quit when his nose was broken. He then took up sambo – a fusion of karate and traditional Soviet wrestling – despite his parents' misgivings. Essen suggests sambo gave the tyro thug discipline, raised him far beyond the hooligans of the courtyard, and helped to transform him into a driven and ambitious adolescent.

Eventually, he was able to join the KGB. He was posted to Dresden, where he had little to do but watch from the sidelines as his beloved Soviet Union withered and died. Then, back in St Petersburg, he made himself useful to city boss Anatoly Sobchak, a man who has often been praised, but a figure whom Gessen excoriates. This was when Putin mastered the politics of greed and power, which he deployed with brutal efficiency.

From then on his rise was smooth and pretty well documented. So it is the early chapters of Gessen's book that are the most fascinating. Her portrait of the obscure young Putin is of a cold, contained youth in whom sentiment and feeling were reserved for institutions rather than people.

Gessen's book is by no means flawless. She can be nippy, not just about Putin, but about other writers. Did she really need to sneer at Anna Politkovskaya – like her, a brave woman journalist, a writer who did much to expose what was going on in Chechnya in the 1990s – as an author who spent much of her life, "writing excessively researched and confusing pieces"? Indeed the phrase "excessively researched" suggests Gessen might have a disdain for the painstaking process of research as opposed to actual writing, though to be fair I can see little evidence of such disdain in this book.

As she charts Putin's later career, covering outrages, mass bombings, disasters and even small wars, Gessen never once lets Putin off the hook of complicity. Her book is, among many other things, the indictment of a mass murderer. Unfortunately, Putin is also a supremely able politician. His methods might bypass most humane and democratic values, but they are terrifyingly effective. He is the hardest of hard men, and he is not, on any account, to be underestimated. Russians know and understand him. The rest of the world must remain very wary of him.

The Man Without A Face: The Unlikely Rise Of Vladimir Putin

By Masha Gessen

Granta, £20