At least 13 are feared dead and many more were injured after terrorist bomb attacks on the main airport and Metro system in Brussels.
The coordinated attacks came as the Belgian capital was on a high level of alert following the arrest of Paris atrocity suspect Salah Abdeslam in the city last week.
There were unconfirmed reports of shots being fired and shouting in Arabic at the airport in what Belgian media said was a suicide attack.
At least one explosion hit the Metro system near a station close to European Union buildings.
On the Metro, traveller Evan Lamos tweeted a picture of passengers climbing from his train into the tunnel, saying: "We are being evacuated from the back of the Metro, between Schuman and Maelbeek.
"Smoke in the tunnel as we evacuate."
The picture was reminiscent of images after the July 7 attacks in London.
Images on social media showed the injured being treated in the street.
As well as the airport, the whole Metro system was closed.
The first news of the attack came when the airport was rocked by a double blast with reports of up to 10 dead and many more injured.
Jef Versele, 40, from Ghent, Belgium, was at the airport when he heard the two explosions.
"I was on my way to check in and two bombs went off - two explosions," he said.
"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 explosion was close to the American Airlines desk.
Images on social media showed shattered windows and smoke rising from an airport building.
People could be seen fleeing in terror in video footage shot from an airport car park.
Footage from inside the building showed a scene of devastation with ceiling tiles strewn across the floor and suitcases abandoned.
Passengers were led onto the tarmac and travellers were urged to stay away from the airport.
Sky News Middle East correspondent Alex Rossi, who was at the airport en route for Tel Aviv, told the channel people were "dazed and shocked".
Mr Rossi, who had checked his luggage through the main baggage area and was at a departure gate, said he thought he was "fairly close" to the explosions but he could not be sure.
He told Sky News as he was being ushered out of the airport along with other passengers: "We felt the walls of the building rock. Dust came down from the ceiling."
Brussels Airport, which handles serving 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.
"Don't come to the airport - airport is being evacuated. Avoid the airport area. Flights have been cancelled.
"All airport operations have been suspended until further notice. Follow the information on our website.
"Passengers that are still located in other areas in the airport are asked to remain calm and wait for further information."
Prime Minister David Cameron called a meeting of the Government's 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."
Foreign secretary Philip Hammond, speaking in Downing Street, said: "It's an ongoing incident. We are in contact with the Belgian authorities and we will give them every assistance we can."
Security has been increased at London Gatwick following the explosions in Brussels, an airport spokesman said today.