Many people have been killed or seriously injured in terrorist attacks at Brussels international airport and a city metro station, Belgium's PM says.
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 Maelbeek metro station. The airport and whole transport system are closed.
The attacks come four days after Salah Abdeslam, the main fugitive in the Paris attacks, was seized in Brussels.
Belgium has now raised its terror threat to its highest level.
Prime Minister Charles Michel called the attacks "blind, violent and cowardly", saying they were a "tragic moment in our country's history. I would like to call on everyone to show calmness and solidarity".
The Belgian prosecutor said it was "probably a suicide bomber" who struck the airport.
European Union president Donald Tusk said: "These attacks mark another low by the terrorists in the service of hatred and violence."
Belgian broadcaster VRT said 13 people were killed at the airport, with at least 35 people severely injured.
Some reports from the metro station speak of at least 10 deaths, but again there is no official confirmation.
The Belga news agency reports that shots were fired and shouts in Arabic were heard before the two explosions at the airport.
mages 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 committeelater 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 a Starbucks store 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 confirmed details yet about the metro blast.
It occurred shortly after 08:00 GMT during the rush hour at Maelbeek station, with TV images showing smoke billowing from the entrance.
Alexandre Brans, told AP: "The metro was leaving Maelbeek station when there was a really loud explosion. It was panic everywhere. There were a lot of people in the metro."
Fire brigade spokesman Pierre Meys told AFP: "Most of the wounded have been evacuated. The scene is rather chaotic."
The station is close to EU institutions. European Commission, the EU's executive arm, has told employees to stay indoors or at home. All meetings at EU institutions have been cancelled.
Ryan McGhee, a catering worker at a college in Brussels, told the BBC: "The entire city is in lockdown. People are calm at the moment but the atmosphere is tense."
Belgium is sending 225 extra troops to Brussels following the explosions, Belga said.
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.