BBC News Europe producer tweets:

People in Brussels have been writing peace messages on the ground at the Place de la Bourse. 

BBC Monitoring

Police in Brussels are searching for terror suspects in the city's Schaerbeek district, Belgian broadcaster RTBF reports, external. 

Image source, AFP/Getty

Buzzfeed interviewed a Belgian counter-terrorism official, external last week - before the Brussels attacks - who said police resources in the country were already stretched to the limit.

He said virtually every police detective and military intelligence official in the country were focused on international jihadi investigations.

“We just don’t have the people to watch anything else and, frankly, we don’t have the infrastructure to properly investigate or monitor hundreds of individuals suspected of terror links, as well as pursue the hundreds of open files and investigations we have,” the official, who spoke on condition of anonymity because he was not authorised to speak to the media, said.

“It’s literally an impossible situation and, honestly, it’s very grave.”

US President Barack Obama has sent a message of support to the people of Brussels, saying: "We can and we will defeat those who threaten the safety and security of people all around the world."

Image source, AP

BBC News Europe reporter Gavin Lee spoke to survivors after the explosions at Zaventem Airport:

"People were running over others that had fallen, I couldn't breathe. I can't believe I'm alive" 15-year-old Antoine tells me, as he walks with his school friends in a line of hundreds being escorted on foot and by bus from to a sports hall, now a makeshift reception centre.

Three funeral hearses pass, heading towards the ruins of the departure hall, people sob at the sight of them. 

Several tourists are asking police what they should do now, where is safe to go - and when the airport might reopen. 

The police are confused too, but offer what little information they have, telling people to head to a reception centre. 

An officer is repeatedly shouting: "Quickly, keep moving, evacuate... this is serious."     

Image source, AFP

Journalist Simon Marks was near Maelbeek Station when the explosion occurred.

He told the BBC: "I was walking by the building when the bomb went off. I heard the noise and saw the smoke from the station. I headed for the entrance. The police hadn't cordoned it off at that point. Clearly, people were injured. There were no paramedics, just a few policemen. 

"A local hotel brought out towels and sheets for rudimentary dressage. About 10 or 15 minutes later paramedics arrived, police cordoned the area and the press were pushed back. There were lots of shocked bystanders in tears. 

"It's eerily quiet now. Roads are shut off. The frenzy of police cars has now calmed."

Image source, Getty Images

BBC Europe Correspondent Chris Morris writes...

European security experts had been braced for another attack for months. But it is always a huge shock when it actually happens.

If this was "revenge" for the arrest of Salah Abdeslam on Friday, it will be a source of considerable concern that a functioning terrorist network was able to respond so quickly and with such devastating effect.

It is possible that a cell linked to Abdeslam brought forward the timing of a future attack because they thought he might blow their cover.

Either way, it shows how advanced the planning was in terms of logistics, explosives, weapons and people willing to carry out such attacks on civilian targets.

The priority now will be to ensure that anyone else who poses an imminent threat to the public is apprehended as soon as possible. But it is clear that there are still huge gaps in intelligence and Brussels is seen as a soft target.

In the words of France's President Hollande, the response from Europe will need to be "calm, lucid and determined" – and it will have to last for a long time.

Belgian police say that Brussels Midi and Brussels North stations will reopen at 16:00 local time (15:00 GMT). Brussels Central will remain closed.

This cartoon responding to the attacks has been widely shared on social media.

Non-essential staff have been evacuated from the Tihange nuclear plant near Liege at the request of the country's authorities, its French operator Engie says. 

A spokesman told Reuters that key staff remained on the site to continue operating the facility.  

Image source, EPA

Image source, AP

The mayor of Brussels says 20 people have died and 106 were injured in the blast on the metro train.

Yvan Mayeur told a news conference that 17 people were severely injured.

Public broadcaster VRT says at least 34 people were killed in total, at the metro and the airport.

The Belgian army's explosives is carrying out a controlled explosion of a suspicious package at Brussels airport, Belgium's crisis centre reports. 

Earlier, a controlled explosion was carried out close to the Maelbeek metro station folllowing the attack there.

A spokeswoman for British Prime Minister David Cameron has confirmed that one British national is known to have been injured in the bomb attack on Brussels airport. The identity of the person is not known.

Brussels transport company STIB has given more details about the metro blast at Maelbeek station, according to Belgian broadcaster RTBF. 

A three-carriage train was just leaving the station in the direction of Arts-Loi, the next stop a short distance away, when the blast happened, RTBF said. 

The bomb was apparently detonated in the middle carriage, which was running along the platform at the time. The driver immediately halted the train and evacuated the first and last carriages.

You can read more here about what we know so far.

Police have found an unexploded suicide vest at Brussels airport, according to Belgian broadcaster VTM.

Earlier reports said a Kalashnikov rifle was also found.

Image source, AFP/Getty

Beatrice Delvaux, chief editor at Belgian daily Le Soir, has written a powerful editorial for her paper's website (in French). She writes:

"The noise is continuous. It comes out from everywhere, crossing the street like an open wound. 

"Ambulances, fire engines, police vans, unmarked cars with flashing blue lights scream as they go. People stop, look, in a daze: their eyes are empty. 

"They know all this is true. They know too that they knew: this was going to happen, this was bound to happen. 

"It's the sadness above all that is infinite; it seeps from the paving stones and trickles from the pavements."

Read the full piece here, external.

A spokesman for John Kerry, the US Secretary of State, said Mr Kerry was "closely monitoring the situation and extends his condolences to all those who have fallen victim today."

He said: "State Department officials are in touch with our embassy in Brussels. The embassy released a security message this morning, recommending that individuals shelter in place and avoid all public transportation. 

"US citizens are urged to be aware of local events, follow local authority instructions and to monitor local media for further developments. 

"Our embassy will continue to update with further information when available. We continue to work to determine the status of all American citizens in Brussels.

"The United States stands with people of Belgium. We are ready to support the investigation as appropriate."

Video footage filmed by a passenger shows the immediate aftermath of the bomb attacks at Brussels airport. The terminal is filled with dust and smoke and people are seen running and crouching low to the ground.

There is no audio.