A suicide bomber blew himself up at Brussels airport on Tuesday killing at least 11 people and a further blast tore through a rush-hour metro train in the capital shortly afterwards, claiming 10 lives, according to public broadcaster VRT.
A witness said he heard shouts in Arabic shortly before two blasts struck the packed airport departure lounge. Pictures on social media showed smoke rising from the terminal building through shattered windows and passengers fleeing down a slipway, some still hauling their bags.
All public transport in Brussels was shut down, as it was in London during 2005 militant attacks on the underground that killed 52. A further 225 soldiers were sent into the city and the Belgian Crisis Centre, clearly wary of a further incident, appealed to the population: "Stay where you are".
The blasts at the airport and metro station occurred four days after the arrest in Brussels of a suspected participant in November militant attacks in Paris that killed 130 people. Belgian police had been on alert for any reprisal action.
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 toward where the explosion came from and there were people coming out looking very dazed and shocked."
Alphonse Youla, 40, who works at the airport, said 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 mangled," he said, his hands covered in blood.
The blasts triggered concern across western Europe with Britain and France calling emergency security meetings. The Dutch military strengthened security at airports and borders and Britain stepped up police presence at key locations including transport hubs.
Video showed devastation inside the departure hall with ceiling tiles and glass scattered across the floor. Some passengers emerged from the terminal with blood spattered over their clothes. Others sat wrapped with blankets.
A witness said the blasts occurred at a check-in desk.
Emergency services (L) at the scene of explosions at Zaventem airport near Brussels, Belgium, March 22, 2016. Reuters/Francois Lenoir
Belga news agency cited the fire brigade as saying 11 were killed at the airport, but there was still some uncertainty about casualties.
The metro station hit by the explosion was Maelbeek, close to European Union institutions.
The VRT broadcaster carried a photograph of a metro carriage at a platform with doors and windows completely blown out, its structure deformed and the interior mangled and charred. It said ten were killed in the blast.
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.
Flights canceled, passengers evacuated
Belgian Prime Minister Charles Michel said on his twitter feed: "We are following the situation minute by minute. Our priority concern is for the victims and those present in the airport.
"For now, we are asking everyone to avoid all movement."
British Prime Minister David Cameron, whose country is also on a high security alert, expressed shock over the attack. "We will do everything we can to help."
Brussels airport said it had canceled all flights until at least 6 a.m. (0500 GMT) on Wednesday and the complex had been evacuated and trains to the airport had been stopped. Passengers were taken to coaches from the terminal that would remove them to a secure area.
All three main long-distance rail stations in Brussels were closed and train services on the cross-channel tunnel from London to Brussels were suspended.
Police did not give any confirmation of the cause of the blast. But there has been a high state of alert across western Europe for fear of militant attacks backed by Islamic State, which claimed responsibility for the Paris attack.
While most European airports are known for stringent screening procedures of passengers and their baggage, that typically takes place only once passengers have checked in and are heading to the departure gates.
Although there may be discreet surveillance, there is nothing to prevent member of the public walking in to the departure hall at Zaventem airport with heavy baggage.
Following an attempted ramraid attack at Glasgow Airport in 2007, several airports stepped up security at entrances by altering the pick-up and drop-off zones to prevent private cars getting too close to terminal buildings.
European stocks fell after the explosions, particularly travel sector stocks including airlines and hotels, pulling the broader indices down from multi-week highs. Safe-haven assets, gold and government bonds rose in price.
French citizen Salah Abdeslam, the prime surviving suspect for November's Paris attacks on a stadium, cafes and a concert hall, was captured by Belgian police after a shootout on Friday.
Belgium's Interior Minister, Jan Jambon, said on Monday the country was on high alert for a revenge attack.
"We know that stopping one cell can ... push others into action. We are aware of it in this case," he told public radio.