AN international manhunt is underway today for one of three bombers who brought carnage to Brussels in terror attacks at an airport and a metro station, which left 34 dead and almost 200 injured, including two Britons.

The Belgian capital was in lockdown as local police issued a wanted notice for a man, pictured wearing a hat and a light-coloured jacket and pushing a luggage trolley through the departure lounge. He was pictured in CCTV footage with two other suspects, believed to have died in the explosions.

A series of police raids were mounted across Belgium, leading to the discovery of an explosive device containing nails, chemical products and a so-called Islamic State(IS) flag. The terror group claimed responsibility for the atrocities.

State prosecutor Frederic Van Leeuw said forensic operations would last well into the night. “Various operations are ongoing across the country and several witnesses have been heard,” he explained.

"Several explosions have been heard. They are due to bomb squad activity upon the discovery that the suspects might have left explosives behind. And this could continue," added Mr Leeuw.

As security was stepped up at transport hubs across Europe, David Cameron, who branded the attacks “appalling and savage”, chaired a meeting of Whitehall’s emergency Cobra committee.

The Prime Minister said Britain had offered Belgium every support. Flags were being flown at half-mast across UK Government buildings, including a Belgian flag over Downing Street.

“We need to stand together against these appalling terrorists and make sure they can never win,” declared Mr Cameron, who is expected to make a Commons statement today.

In Edinburgh, the Scottish Government’s resilience committee also met. Nicola Sturgeon said there was no specific threat to Scotland but urged the public to be vigilant. The First Minister said she had spoken to the Belgian Ambassador to "to convey Scotland's sympathy, support and solidarity for the people of Belgium".

The police presence was intensified at the UK’s main airports and ports as well as at Eurostar and on London’s Underground.

Phil Gormley, Police Scotland Chief Constable, called on the public to be “sensibly vigilant” and said security was being reviewed at Scotland’s transport hubs with increased uniformed patrols at Glasgow and Edinburgh airports.

The Foreign Office has urged Britons not to travel to Brussels unless it is absolutely necessary. Those in the Belgian capital have been advised to avoid crowded places.

Charles Michel, the Belgian Prime Minister, said the attacks were “cowardly” and that it was a “black day” for his country. “I would like to call on everyone to show calmness and solidarity," he added.

As night fell on Brussels, he lit a candle at a vigil at Place de la Bourse, the city's stock exchange building.

Later in a televised address, King Philippe said he and Queen Mathilde "share the pain" of all those who had suffered in the attacks. But he called on Belgians to stay "confident" in the face of terror.

Belgium has raised its terrorism alert to its highest level. Three days of national mourning have been declared.

The terrorists first struck at 7.20am UK-time. Twin blasts hit Zaventem airport. Eleven people were killed. Reports suggested some survivors of the first explosion fled only to be hit by the second.

The target of the attack was an American Airlines check-in desk. A suitcase bomb exploded. Local reports said shouts in Arabic were heard as gunshots were fired. Two Kalashnikov rifles and an unexploded bomb-belt were later found.

TV footage from inside the airport building showed a scene of devastation with ceiling tiles strewn across the floor and suitcases abandoned.

Jef Versele, a 40-year-old from Ghent, said "I was on my way to check in and two bombs went off...Everything was coming down; glassware. It was chaos. It was unbelievable. It was the worst thing."

Pierre Meys, a local firefighter, described seeing "war injuries" and noted: “This is the worst thing I've ever seen in my career."

An hour later during rush-hour, another explosion struck Maelbeek metro station near the European Union’s headquarters, as a train started to leave the platform. Twenty people were killed. All EU institutions were closed.

Images of passengers climbing from a train into a smoke-filled tunnel near the metro station were reminiscent of scenes following the July 7 attacks in London.

Local Alexandre Brans said: "The metro was leaving Maelbeek station when there was a really loud explosion. It was panic everywhere. There were a lot of people in the metro."

Some time after the attacks, a third bomb was deactivated at the airport.

So-called IS claimed responsibility for the attacks, saying its members opened fire in the airport and "several" others detonated suicide belts. The terror group said a suicide bomber was also responsible for the metro attack.

The atrocities prompted serious questions about the adequacy of Belgian intelligence with many people blaming the authorities.

The attacks in Brussels occurred four days after the arrest there of Salah Abdeslam, the key suspect of last November’s terror attacks in Paris.

One theory is the atrocities were in revenge for his arrest; another that the cell linked to Abdeslam brought forward the timing of a future attack because its members thought he might blow their cover.

Brussels is regarded by some as the epicentre of terrorist threat in Europe with many local Muslims having travelled to Syria to join up with IS. Brussels was believed to be the logistics centre for the Paris attacks.

The atrocities in Brussels caused shares in airlines and transport firms to plunge into the red as European stock markets were sent reeling.

Leaders across the world condemned the attacks.

US President Barack Obama said they were "outrageous attacks against innocent people" while Russian President Vladimir Putin denounced them as “barbaric”.

In a joint statement all 28 EU leaders said the bombings were an "attack on our open democratic society" while Francois Hollande, the French premier, said: "The terrorists have struck Belgium but it is Europe that was targeted."

Meantime, Celtic player, Dedryck Boyata, who is a Belgian international, tweeted: "My thoughts and prayers are with the victims #zaventem."