Several explosions have struck Brussels airport and the metro system, causing at least 13 deaths, Belgian media say.
Two blasts tore through the departures area of Zaventem airport shortly after 08:00 local time (07:00 GMT).
An hour later, an explosion hit Maalbeek metro station, close to the EU institutions. The airport and whole transport system have been closed.
The attacks come four days after Salah Abdeslam, the main fugitive in the Paris attacks, was seized in Brussels.
The Belgian government says there have been casualties at the airport but has given no numbers. The cause of the explosions has not been confirmed.
Belgium has now raised its terror threat to its highest level. Prime Minister Charles Michel tweeted: “For the moment, we are asking people to stay where they are.”
Belgian broadcaster VRT said that in addition to the dead at the airport, at least 35 people were severely injured.
The Belga news agency reports that shots were fired and shouts in Arabic were heard before the two explosions. Some reports say it was a suicide attack.
Images on social media showed panic as people fled the airport.
Rail transport to the airport has been halted and people have been told not to come. All flights have been diverted. Eurostar has cancelled all trains to and from Brussels.
Security has been stepped up at Gatwick and Heathrow airports and the UK Foreign Office has advised British nationals to avoid crowded areas in Belgium. UK PM David Cameron will chair a meeting of the Cobra response committee later on Tuesday morning.
France has stepped up security. President Francois Hollande has held a cabinet meeting to discuss the Belgian explosions. There is also extra security at Dutch airports.
Some reports say at least one of the explosions at Zaventem was close to the American Airlines check-in area but this has not been confirmed.
Zach Mouzoun, arriving on a flight from Geneva, told France's BFM television: “It was atrocious. The ceilings collapsed. There was blood everywhere, injured people, bags everywhere. We were walking in the debris. It was a war scene.”
There are few details yet about the metro blast, but there are some reports of casualties.
It occurred shortly after 08:00 GMT during the rush hour at Maalbeek station, with TV images showing smoke billowing from the entrance.
Alexandre Brans, told AP: “The metro was leaving Maalbeek station when there was a really loud explosion. It was panic everywhere. There were a lot of people in the metro.”
The European Commission, the EU's executive arm, has told employees to stay indoors or at home. All meetings at EU institutions have been cancelled.
Belgium's Interior Minister Jan Jambon had said on Monday that the country was on the highest level of alert for possible revenge attacks after the capture of Salah Abdeslam.
He told Belgian radio: “We know that stopping one cell can… push others into action. We are aware of it in this case.”
Zaventem airport is 11km (7 miles) north-east of Brussels and dealt with more than 23 million passengers last year.