Belgium's crisis centre has urged people in Brussels to stay safe and stay indoors. 

Schoolchildren have been urged to stay at school and office workers have been asked not to go out. 

The last time Brussels went into lockdown was in November, in the aftermath of the Paris attacks. The country-wide alert level is again at its highest.

BBC Monitoring

The Flemish-speaking Free University of Brussels (VUB) has said it has received a "specific threat" to its Etterbeek campus in the city and has advised students and staff to remain indoors or return to university buildings immediately.

Access to the campus is being restricted by security personnel, the university said on its website.  

Evan Lamos was travelling to work when the metro train he was on was evacuated. He told BBC Radio 5 Live that his ears popped when the blast occurred, followed by announcements telling passengers to remain calm.

• Two bombs went off at Brussels airport shortly before 08:00 (07:00 GMT) close to two check-in desks in the departure hall.
• At least 11 people were killed and another 81 were wounded at the airport, Belgian health officials say.
• Another bomb went off at around 09:11 at Maelbeek metro station in the heart of Brussels' European quarter, close to the European Union buildings.
• The metro operator says 15 people died and another 55 were wounded at the station.
• All three blasts have been described as "terrorist attacks" by Belgium's federal prosecutor; one of the airport attacks is being blamed on a suicide bomber.
• Belgian Prime Minister Charles Michel has called the attacks a "black moment" for his country.
• Public transport in Brussels has ground to a halt and security has been stepped up and key transport locations in Paris, London, New York and elsewhere.
• Belgian anti-terror police are conducting house searches in Brussels, according to Belgian state broadcaster RTBF.

Speaking to reporters in Whitehall, Boris Johnson, the mayor of London, said: "It looks as though there may be one UK casualty, as I'm sure you are aware, though the details of that are very sketchy. 

"The most important thing to get across is we know of no reason to think that there is any read across from what's happened in Brussels to events here in this city. 

"We have no intelligence to suggest there is any immediate threat, but as a precaution and for the purposes of reassurance, there has been a stepping up of the presence of police at major airports." 

Mr Johnson did not say whether the person had died or was injured, but the BBC's Norman Smith reports that there are understood to be no British fatalities.

Police are carrying out house searches in Brussels, RTBF reports. Suspects believed to be linked to the attacks are being sought, the broadcaster said.

Speaking at a press conference, French President Francois Hollande said France and Belgium were "linked with horror" following the attacks in Paris in December and now in Brussels.

Mr Hollande said he had assured the Belgian government of France's full support and said the "war against terrorists" should be carried out through the "whole of Europe using all the necessary means, especially sharing intelligence information".

He said the French government had taken measures to bolster security at borders and at its ports. 

The hashtag "ikwilhelpen" (I want to help) has become prominent across social media in Belgium. Twitter users in Belgium are urging others to use it if they still have space in their cars or need a lift home.

The Belgian health ministry has said that 11 people were killed and 81 were wounded in the two blasts at Brussels airport. 

Earlier, the Brussels metro operator said that 15 people died in the attack at Maelbeek station and 55 were wounded.

That brings the death toll for both locations to 26 and the number of wounded to 136.

Belgian state broadcaster RTBF is reporting that a Kalashnikov rifle has been found in Brussels airport departure hall.

The rifle, recognisable by its distinctive design, was also used by the Paris attackers.

More: Kalashnikov AK-47 in 60 seconds

King Philippe and Queen Mathilde have issued a statement in response to the bombings. "Distressed by the odious attacks on Brussels-National and the Brussels metro. Our thoughts are with the victims, families and emergency services."

Speaking at a press conference, French President Francois Hollande condemned what he called the "heinous" attacks in Brussels today.

He said attacks had been directed at Belgium, but it was Europe that had been targeted and the whole world that should be concerned about the attacks. 

He said there was a "global threat which necessitates a global response".

Richard Westcott
Transport correspondent

The information we have so far suggests the bombings at Brussels airport happened before any security checks, in the public drop-off area. So it doesn't look like a security lapse at the scanning area. 

There is a limit to what you can do at airports to stop this sort of attack. Prevention is more reliant on getting intelligence beforehand. American Airlines has told us they were not the nearest check-in desk.

Public transport networks in Brussels are closed and security has been tightened across Europe. 

Here is a list of transport disruption and advice for travellers:

• The Brussels airport authority says all airport operations are suspended, external.

• All buses, trains and trams have stopped running, external and all metro stations have been. closed, the city's transport authorities say.
• The Brussels Crisis Center has told people to stay where they are, external and not to travel.
• Belgium has increased its terror threat level to four, external for the whole country.
• Security has been stepped up at nuclear facilities in Belgium.

  For more information, click here. 

The French President tweeted: "I express my complete solidarity with the Belgian people. Through the attacks on Brussels, it is Europe as a whole that has been hit."

BBC Monitoring

The Brussels attacks bear the hallmark of the Islamic State (IS) militant group, according to Belgian jihadist analyst Pieter Van Ostaeyen.

He told Belgium's Het Nieuwsblad newspaper he was surprised by the nature of the operation: "I had expected that something would happen but not on this scale. This is really highly coordinated," he said. 

Mr Van Ostaeyen said the attacks were most likely a response to the counter-terror operation in Brussels last week and that they were probably "pulled together at very short notice".

Speaking at a press conference, Belgian Prime Minister Charles Michel called the attacks "blind, violent and cowardly", saying they were a "tragic moment in our country's history".

He called on the people of Belgium to "show calmness and solidarity".  

The Brussels metro operator has confirmed a provisional death toll for the attack at Maelbeek station: 15 dead, 55 wounded.