At least 34 people were killed and 170 injured after coordinated bomb attacks on the main airport and the Metro system brought terror to Brussels.
The attacks, condemned as "blind, violent and cowardly" by Belgian Prime Minister Charles Michel, came after the arrest in the city last week of terror mastermind Salah Abdeslam, who plotted November's Paris atrocity from the notorious Molenbeek suburb of Brussels.
Responding to the attacks, French Prime Minister Manuel Valls said: "We are at war. In Europe we have been subjected to acts of war for several months."
Belgian foreign minister Didier Reynders warned: "We fear that people are still at large."
David Cameron hit out at "inappropriate" attempts to link the terror attacks to the debate on Britain's future in the EU, adding that the countries of Europe need to "stand together against these appalling terrorists and make sure they can never win".
A suicide bomber struck first near the American Airlines desk at Brussels airport, with local media reporting that shots were fired amid shouting in Arabic.
The explosion on the Metro was near European Union buildings and the US embassy.
As the city went into lockdown:
:: Broadcaster RTBF reported 20 people were killed following the explosion at Maelbeek metro station, while 14 died in a suicide attack at the airport
:: The number of injured people in both attacks is believed to be 170
:: Downing Street said one British national is known to have been injured at the airport
:: Police forces across the UK increased their presence at key locations as a precaution
:: Britons in Brussels were advised by the Foreign Office to avoid crowded places and public transport
First news of the attacks came when the airport was rocked by a double blast around 7am UK time.
Jef Versele, 40, from Ghent, told the Press Association: "I was on my way to check in and two bombs went off - two explosions.
"I didn't see anything. Everything was coming down. Glassware. It was chaos. It was unbelievable. It was the worst thing."
He added: "People were running away. There were lots of people on the ground. A lot of people are injured."
Mr Versele was two or three storeys above the source of the explosion but he said many people around him were hurt.
"The bomb was coming from downstairs. It was going up through the roof. It was big.
"About 15 windows were just blown out from the entrance hall," he added.
The attack on the Metro came about an hour later.
Evan Lamos tweeted a picture of passengers climbing from his train into the tunnel near Maelbeek station, saying: "We are being evacuated from the back of the Metro.
"Smoke in the tunnel as we evacuate."
The picture was reminiscent of images after the July 7 attacks in London.
Other images showed the injured from the Metro being treated in the street while at the airport people could be seen fleeing in terror in video footage shot from an airport car park.
Footage from inside the airport building showed a scene of devastation with ceiling tiles strewn across the floor and suitcases abandoned.
Responding to the attacks, Prime Minister Michel said: "A series of additional security measures have been taken at this moment.
"Military reinforcements have been deployed, reinforcement of border controls and specific restrictions on public transport have been put into place at this time.
"I also would like to say that in this tragic and dark moment for our country, more than ever I call everyone to show calm but also solidarity.
"We are faced with a challenge, a difficult challenge, and we have to face this challenge being united, showing solidarity and being together."
Speaking to reporters in Whitehall, Mayor of London Boris Johnson, who used to live in Brussels, said Britain "must show maximum possible solidarity with the people of Brussels and Belgium".