Explosions tore through the departure hall of Brussels airport on Tuesday morning killing 34 persons and injuring over 100 others, the Belgian news agency Belga said.

The agency said shots were fired and there were shouts in Arabic shortly before the explosions.

The blasts 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.

Small blasts took place close to a central Brussels metro station that has been sealed off by police on Tuesday, a Reuters journalist said, and local media said it appeared to be the work of the police bomb squad.

Social media showed pictures of smoke rising from the departure hall where windows had been shattered by the blasts. Passengers were seen running away down a slipway.

Sky News television's Alex Rossi, at the scene, 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."

"The thinking here is that it is some kind of terrorist attack - that hasn't been verified by any of the authorities here at the airport."

Video showed devastation inside the departure hall with items scattered across the floor.

Belgian media said rail traffic to the airport was suspended.

Brussels airport said it had cancelled all flights and the complex had been evacuated.

"Don't come to the airport - airport is being evacuated. Avoid the airport area. Flights have been cancelled," the airport said..

Brussels Airport serves over 23 million passengers a year.

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 possible revenge attack following the capture of 26-year-old Abdeslam.

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

"The thinking here is that it is some kind of terrorist attack - that hasn't been verified by any of the authorities here at the airport."

The legs of some of those killed at Brussels airport on Tuesday were shattered, as if the blast came from a piece of luggage nearby, an airport worker who helped carry their bodies said.

Alphonse Youla, 40, who works at Zaventem airport luggage security said that before the first bomb went off, he heard a man shouting something in Arabic.

"Then the tiled ceiling of the airport collapsed. I helped carry out five dead, with their legs destroyed, as if the bomb came from a piece of luggage" he told reporters, his hands covered in blood. "It's from the people I carried out".

Public broadcaster VRT said 81 people had been wounded at airport, many in the legs, suggesting a bomb in a bag on the floor.

The federal prosecutor told a news conference one of the two explosions at the airport was likely to have been caused by a suicide bomber.

Hundreds of passengers who had not been injured were leaving the airport by a side exit, many in tears, some covered with blankets. Many were being evacuated by bus.

Passenger Paolo Saraca Volpini said an airport announcer's voice came over the public address system about a quarter of an hour after the blasts, his voice breaking with emotion, and said in several languages "we are experiencing an attack" and asking people in Terminals A and B to stay where they were.

One passenger, who had already cleared the security checks when the bombs went off, said that after the explosions, passengers already inside the airport panicked and started running in search of shelter.

"People were taking cover in shops and where they could. We managed to get on the plane, but it did not take off and then we were escorted out to busses to leave," said Sylwia Czerska, who was on her way to Geneva.

Witnesses said the first explosion took place close to Belfius bank, near the lifts in the left wing of the departures hall. The second was stronger and was closer to the Starbucks cafe which is in the centre of the hall.

Samir Derrouich, who works for the Autogrill restaurant at the airport, said of the explosions: "They were almost simultaneous. It was an apocalypse".

Video showed devastation inside the departure hall with items scattered across the floor

French investigator Francois Molins told a news conference in Paris on Saturday that Abdeslam, a French citizen born and raised in Brussels, admitted to investigators he had wanted to blow himself up along with others at the Stade de France on the night of the attack claimed by Islamic State; but he later backed out.

Furthermore, another explosion was heard at Maelbeek metro station in Brussels, close to the EU institutions, Belgian broadcaster RTBF said, after explosions ripped through the departure hall at Brussels airport.

Metro operator STIB announced on Twitter that the metro was closing.

European shares fell in early trading on Tuesday, with travel and leisure stocks leading the market lower after explosions hit the airport in Brussels.

Explosions tore through the departure hall of Brussels airport on Tuesday morning, killing one person and injuring several others, the Belgian news agency Belga said. The agency said shots were fired and there were shouts in Arabic shortly before the explosions.

The STOXX Europe 600 Travel and Leisure index fell 2.2 percent, the top decliner, with shares in easyJet, Ryanair, Accor, InterContinental Hotels and IHG down by 2.4-4.6 percent.

The pan-European FTSEurofirst 300 index was down 0.7 percent, while Belgium's benchmark share index was down 0.5 percent

UK Prime Minister David Cameron said he would chair a crisis response meeting following explosions in Brussels on Tuesday.

"I am shocked and concerned by the events in Brussels. We will do everything we can to help," Cameron said on Twitter, adding that he would chair a meeting of the COBRA response committee.

"I will be chairing a COBRA meeting on the events in Brussels later this morning," Cameron added.

Meanwhile Dutch police stepped up security patrols at airports and tightened checks at borders after Tuesday's attacks in neighbouring Belgium, the security agency said.

Travellers passing through Amsterdam's Schiphol Airport reported delays and a heavy police presence. Security agencies declined to give details of any further measures taken, but maintained the national threat level at "substantial", one notch below the highest.

Flights were diverted from Brussels to Amsterdam following attacks at the Belgian capital's airport earlier on Tuesday in which at least 13 people are believed killed. Trains heading south to Belgium were subject to indefinite delays, Dutch state railways said. Also, Eurostar cancelled trains running to and from Brussels on Tuesday after explosions in the city.

"No trains are currently running to or from Brussels Midi," the high speed rail service said on its Twitter feed. "Brussels customers are advised to postpone, and not come to station".

Police in Denmark, Sweden and Finland have also stepped up security at airports and public places following the explosions in Brussels on Tuesday.

Danish police said they had increased patrols at Copenhagen airport and other key points in the city following the deadly explosions at Brussels airport and a metro station in the city.

"We are aware of what has happened in Brussels. Therefore you will see more police in the airport and at key points in Copenhagen," Danish police said on its official Twitter page.

Danish authorities have been on high alert since two people were killed in shooting attacks on a free speech event and a synagogue in Copenhagen in February last year.

Police in Sweden said they had reinforced their presence at airports and taken increased security measures at other public places. Finnish Interior Minister Petteri Orpo said on Twitter "Finnish security officials have increased monitoring at Helsinki-Vantaa airport"

Meanwhile British police said they would step up their presence at key locations across the country, including transport hubs, after explosions in Brussels on Tuesday.

Mark Rowley, Britain's most senior counter-terrorism officer, said the move was precautionary and did not result from any specific intelligence.

"In London specifically, the Metropolitan Police Service has mobilised additional officers, who will carry out highly visible patrols at key locations around the capital including the transport network," Rowley said in a statement.

"We are in close liaison with the Belgium authorities and will continue to monitor the situation."

In the context of the Brussels blasts France has decided to deploy 1,600 additional police officers to bolster security at its borders and on public transport following the deadly blasts in Brussels on Tuesday, French Interior Minister Bernard Cazeneuve said.

A total of 400 additional police officers will boost security in the greater Paris area, and military patrols will be refocused on public transport sites, the minister said after an emergency meeting with President Francois Hollande.

Cazeneuve said the country continued to face an "extremely high" security threat four months after the Islamist militant attacks in Paris in November that killed 130 people.

"After this morning's attacks in Brussels, I decided to deploy 1,600 extra police officers at different points across the country, at border checkpoints, and also on air, sea and rail transport infrastructure," Cazeneuve told reporters.

The attacks in Brussels today appear to be aimed not just at Belgium but at the entire European Union and its freedoms," German Interior Minister Thomas de Maiziere said.

"It seems that the clear targets of the attacks - an international airport, a metro station close to EU institutions - indicate that this terrorist attack is not aimed solely against Belgium but against our freedom, freedom of movement, mobility and everyone in the EU," he told a news conference in Berlin.

*This is a developing story