At least 21 people were killed and many more injured after terrorist bomb attacks on the main airport and the Metro system in Brussels.
The coordinated attacks came as the Belgian capital was on high alert following the arrest of Paris atrocity suspect Salah Abdeslam in the city last week.
A suicide bomber struck near the American Airlines desk at the 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 lock down:
:: A fire service spokesman said there were 11 dead at the airport and "around 10" at Maelbeek station, where "there was a very big explosion"
:: Police forces across the UK increased their presence at key locations as a precaution in the wake of the attacks
:: Belgian prime minister Charles Michel tweeted: "For the moment, we are asking people to stay where they are"
:: Brussels Airport, which handles 23.5 million passengers per year, told users on Twitter: "There have been 2 explosions at the airport. Building is being evacuated. Don't come to the airport area"
:: Britons in Brussels were advised by the Foreign Office to avoid crowded places and public transport
:: David Cameron called a meeting of the Cobra emergency committee and tweeted a message of support, saying: "I am shocked and concerned by the events in Brussels. We will do everything we can to help"
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 and left at least 10 dead, according to local media.
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.