Islamic State claimed responsibility for attacks on Brussels airport and a rush-hour metro train in the Belgian capital on Tuesday which killed at least 30 people, a news agency affiliated to the group said.
The coordinated assault triggered security alerts across Europe and drew global expressions of support, four days after Brussels police had captured the prime surviving suspect in Islamic State's attacks on Paris last November.
A witness said he heard shouts in Arabic and shots shortly before two blasts struck a packed airport departure lounge at Brussels airport.
The federal prosecutor said one of the explosions was probably triggered by a suicide bomber.
Belgian media published a security camera picture of three young men pushing laden luggage trolleys through the airport and reported that police suspected them of being the attackers.
They said two were suspected of having blown themselves up while police were hunting the third.
The AMAQ news agency carried the claim of responsibility. "Islamic State fighters carried out a series of bombings with explosive belts and devices on Tuesday, targeting an airport and a central metro station in the centre of the Belgian capital Brussels," it said.
US President Barack Obama led calls of support to Belgian Prime Minister Charles Michel after Brussels went into a state of virtual lock-down.
"We must be together regardless of nationality or race or faith in fighting against the scourge of terrorism," Obama told a news conference in Cuba.
"We can and we will defeat those who threaten the safety and security of people all around the world."
Michel spoke at a Brussels news conference of a "black moment" for his country. "What we had feared has come to pass."
The blasts occurred four days after the arrest in Brussels of a suspected participant in November militant attacks in Paris that killed 130 people.
Belgian police and combat troops on the streets had been on alert for reprisal but the attacks took place in crowded areas where people and bags are not searched.
Transport shut down
All public transport in Brussels was shut down, as it was in London during 2005 Islamist militant attacks there that killed 52.
Authorities appealed to citizens not to use overloaded telephone networks, extra troops were sent into the city and the Belgian Crisis Centre, clearly wary of a further incident, appealed to the population: "Stay where you are".
Later, people were told that mainline rail stations would open at 4pm (1500 GMT) to let commuters head home.
British Sky News television's Alex Rossi, at the airport, said he heard two "very, very loud explosions".
"I could feel the building move. There was also dust and smoke as well...I went towards where the explosion came from and there were people coming out looking very dazed and shocked."
Public broadcaster VRT said police had found a Kalashnikov assault rifle next to the body of an attacker at the airport.
Such weapons have become a trademark of Islamic State-inspired attacks in Europe, notably in Belgium and France, including on Nov 13 in Paris.
An unused explosive belt was also found in the area, the public broadcaster said. Police were continuing to scour the airport for any further bombs or attackers.
Alphonse Youla, 40, who works at the airport, told Reuters he heard a man shouting out in Arabic before the first explosion. "Then the glass ceiling of the airport collapsed."
"I helped carry out five people dead, their legs destroyed," he said, his hands covered in blood.
A witness said the blasts occurred at a check-in desk.
Video showed devastation in the hall with ceiling tiles and glass scattered across the floor.
Some passengers emerged from the terminal with blood spattered over their clothes. Smoke rose from the building through shattered windows and passengers fled down a slipway, some still hauling their bags.
Public broadcaster RTBF said police were searching houses in the Brussels area.
A Crisis Centre spokesman issued provisional figures of 20 killed in the metro and 10 at the airport.
Public broadcaster VRT had said earlier 20 were killed in the train and 14 at the airport.
Many of the dead and wounded at the airport were badly injured in the legs, one airport worker told Reuters, suggesting at least one bomb in a bag on the floor.
Britain, Germany, France and the Netherlands, all wary of spillover from conflict in Syria, were among states announcing extra security measures. Security was tightened at the Dutch border with Belgium.
The blast hit the train as it left Maelbeek station, close to European Union institutions, heading to the city centre.
VRT carried a photograph of a metro carriage at a platform with doors and windows completely blown out, its structure deformed and interior mangled and charred.
A local journalist tweeted a photograph of a person lying covered in blood among smoke outside Maelbeek metro station, on the main Rue de la Loi avenue which connects central Brussels with the EU institutions.
Ambulances were ferrying the wounded away and sirens rang out across the area.