A victim receives first aid by rescuers on March 22, 2016 near Maalbeek metro station in Brussels, after a blast at this station near the EU institutions caused deaths and injuries. Picture: AFP/ Emmanuel Dunand

Brussels - Belgian Prime Minister Charles Michel said Tuesday that the "blind, violent and cowardly" attacks on Brussels airport and the metro system were a "black day" for the country and had left many dead.

"This is a day of tragedy, a black day," Michel said on national television, with the death toll put at 21 so far and expected to rise further.

A suicide bomber blew himself up at Brussels airport on Tuesday killing at least 11 people and a further blast tore through a rush-hour metro train in the capital shortly afterwards, claiming 10 lives, according to public broadcaster VRT.

A witness said he heard shouts in Arabic shortly before two blasts struck the packed airport departure lounge.

Pictures on social media showed smoke rising from the terminal building through shattered windows and passengers fleeing down a slipway, some still hauling their bags.

All public transport in Brussels was shut down, as it was in London during 2005 militant attacks on the underground that killed 52.

A further 225 soldiers were sent into the city and the Belgian Crisis Centre, clearly wary of a further incident, appealed to the population: “Stay where you are”.

The blasts at the airport and metro station occurred four days after the arrest in Brussels of a suspected participant in November militant attacks in Paris that killed 130 people. Belgian police had been on alert for any reprisal action.

British Sky News television's Alex Rossi, at the airport, said he heard two “very, very loud explosions”.

“I could feel the building move. There was also dust and smoke as well...I went towards where the explosion came from and there were people coming out looking very dazed and shocked.”

Alphonse Youla, 40, who works at the airport, said he heard a man shouting out in Arabic before the first explosion.

“Then the glass ceiling of the airport collapsed.”

“I helped carry out five people dead, their legs mangled,” he said, his hands covered in blood.

The blasts triggered concern across western Europe with Britain and France calling emergency security meetings.

The Dutch military strengthened security at airports and borders and Britain stepped up police presence at key locations including transport hubs.

Video showed devastation inside the departure hall with ceiling tiles and glass scattered across the floor. Some passengers emerged from the terminal with blood spattered over their clothes. Others sat wrapped with blankets.

A witness said the blasts occurred at a check-in desk.

Belga news agency cited the fire brigade as saying 11 were killed at the airport, but there was still some uncertainy about casualties.

The metro station hit by the explosion was Maelbeek, close to European Union institutions.

The VRT broadcaster carried a photograph of a metro carriage at a platform with doors and windows completely blown out, its structure deformed and the interior mangled and charred. It said ten were killed in the blast.

A local journalist tweeted a photograph of a person lying covered in blood among smoke outside Maelbeek metro station, on the main Rue de la Loi avenue which connects central Brussels with the EU institutions.

Ambulances were ferrying the wounded away and sirens rang out across the area.

Belgian Prime Minister Charles Michel said on his twitter feed: “We are following the situation minute by minute...For now, we are asking everyone to avoid all movement.”

British Prime Minister David Cameron, whose country is also on a high security alert, expressed shock over the attack. “We will do everything we can to help.”

Brussels airport said it had cancelled all flights until at least 6am (05.00 GMT) on Wednesday and the complex had been evacuated and trains to the airport had been stopped. Passengers were taken to coaches from the terminal that would remove them to a secure area.

All three main long-distance rail stations in Brussels were closed and train services on the cross-channel tunnel from London to Brussels were suspended.

Security services have been on a high state of alert across western Europe for fear of militant attacks backed by Islamic State, which claimed responsibility for the Paris attack.

While most European airports are known for stringent screening procedures of passengers and their baggage, that typically takes place only once passengers have checked in and are heading to the departure gates.

Although there may be discreet surveillance, there is nothing to prevent member of the public walking in to the departure hall at Zaventem airport with heavy baggage.

Following an attempted ramraid attack at Glasgow Airport in 2007, several airports stepped up security at entrances by altering the pick-up and drop-off zones to prevent private cars getting too close to terminal buildings.

European stocks fell after the explosions, particularly travel sector stocks including airlines and hotels, pulling the broader indices down from multi-week highs. Safe-haven assets, gold and government bonds rose in price.

French citizen Salah Abdeslam, the prime surviving suspect for November's Paris attacks on a stadium, cafes and a concert hall, was captured by Belgian police after a shootout on Friday.

Belgium's Interior Minister, Jan Jambon, said on Monday the country was on high alert for a revenge attack.

“We know that stopping one cell can ... push others into action. We are aware of it in this case,” he told public radio.

World leaders condemn Brussels attacks as “barbaric, cowardly”

World leaders express their solidarity with Belgium and condemn as “barbaric” the Brussels airport and subway attacks.

“The despicable attacks in Brussels should bring us together: Solidarity with the victims + determination towards the terrorists.” - German Chancellor Angela Merkel's spokesman Steffen Seibert on Twitter.

“With heart and mind in Brussels, Europe.” - Italian Prime Minister Matteo Renzi on Twitter

“Greece stands in solidarity with the citizens of Belgium and EU. We can't allow fear, religious hatred and racism to prevail in Europe.” - Greek Prime Minister Alexis Tsipras on Twitter

“These attacks against civilians in the centre of Europe carry the signature of cowardice and barbarism ... Europe must fight terrorism together and must defend its democratic values.” - Austrian Chancellor Werner Faymann, Vice Chancellor Reinhold Mitterlehner

“I am shocked and concerned by the events in Brussels. We will do everything we can to help.” - British Prime Minister David Cameron on Twitter

“Today is a black day for #Europe. The horrible events in #Brussels affect us all. We are steadfastly at the Belgians' side.” - German Justice Minister Heiko Maas on Twitter

“All solidarity now with EU, Belgium, #Brussels! Terrorists will never win: Our European values much stronger than hate, violence, terror!” - Merkel's chief of staff Peter Altmaier

“Holding my breath I am close to Belgians in the face of the shocking attack at the heart of Brussels.” - Italian Foreign Minister Paolo Gentiloni on Twitter

“The fight against this evil demands the highest level of active international cooperation.” - Russian President Vladimir Putin

“Abominable attacks in BXL. My thoughts are with those affected and their families. We stand together with Belgium.” - Danish Prime Minister Lars Lokke Rasmussen on Twitter

“It is an attack on the democratic Europe. We will never accept that terrorists attack our open societies.” - Swedish Prime Minister Stefan Lofven

“Terrorism will not be able to defeat us. The unity of EU democrats is and always will be above barbarism and lack of reason.” - Spanish Prime Minister Mariano Rajoy on Twitter

“Once again Europe is under attack. We stand with Belgium. Those using death and violence must and will be defeated.” - Ireland's acting Prime Minister Enda Kenny on Twitter

“News from Brussels is disturbing. The attacks are condemnable.” - Indian Prime Minister Narendra Modi on Twitter

“Terrorism is not a threat to any single country or nation, but to the whole humanity and it is high time that this menace is collectively fought for safeguarding our future generations.” - Pakistan Prime Minister Nawaz Sharif

Reuters - Additional report by AFP and DPA