Bombs exploded at the Brussels airport and one of the city's Metro stations on Tuesday, killing at least 31 people and wounding dozens, as a European capital was again locked down amid heightened security threats.
The Islamic State group claimed responsibility for the attacks and Belgian Police have issued a CCTV picture of a man who is suspected of carrying out the attack at Brussels airport.
The two airport blasts, at least one of them blamed on a suicide bomber, left behind a chaotic scene in the departure lounge as windows were blown out, ceilings collapsed and travellers streamed out of the smoky building.
About an hour later, another bomb exploded on a rush-hour subway train near the European Union headquarters. Terrified passengers had to evacuate through darkened tunnels to safety.
'What we feared has happened,' Belgian prime minister Charles Michel told reporters. 'In this time of tragedy, this black moment for our country, I appeal to everyone to remain calm but also to show solidarity.'
Belgium raised its terror alert to the highest level, diverting planes and trains and ordering people to stay where they were for most of the workday. Airports across Europe immediately tightened security.
'We are at war,' French prime minister Manuel Valls said after a crisis meeting in Paris. 'We have been subjected for the last few months in Europe to acts of war.'
French president Francois Hollande added: 'Terrorists struck Brussels, but it was Europe that was targeted, and it is all the world which is concerned by this.'
European security officials have been bracing for a major attack for weeks, and warned that Islamic State was actively preparing to strike. The arrest Friday of Salah Abdeslam, a key suspect in the November attacks in Paris, heightened those fears, as investigators said many more people were involved than originally thought and that some are still on the loose.
Islamic State claimed responsibility for the Brussels attacks, saying in a posting on the group's Amaq news agency that its extremists opened fire in the airport and 'several of them' detonated suicide belts. It said another suicide attacker struck in the subway.
The posting claimed the attack was in response to Belgium's support of the international coalition arrayed against the group.
Police found and neutralised a third bomb at the airport once the chaos after the two initial blasts had eased, said Florence Muls, a spokeswoman for the airport. Bomb squads also detonated suspicious objects found in at least two locations elsewhere in the capital, but neither contained explosives, authorities said.
Mr Michel said there was no immediate evidence linking the attacks with Abdeslam. After his arrest, Abdeslam told authorities he had created a new network and was planning new attacks.
US president Barack Obama pledged to 'do whatever is necessary' to help Belgian authorities seek justice.
'We stand in solidarity with them in condemning these outrageous attacks against innocent people,' Mr Obama said in Havana, where he was closing a three-day visit.
Certain neighbourhoods in Brussels, like the Molenbeek quarter, have bred extremists and supplied foreign fighters. Plotters linked to the Paris attacks and others have either moved through or lived in parts of the city.
Tuesday's explosions at the airport in the Brussels suburb of Zaventem came shortly after 8am, one of its busiest periods. Belgian health minister Maggie de Block said 11 people were killed and 81 wounded. Eleven people had serious injuries, Marc Decramer, of the Gasthuisberg hospital, in Leuven told broadcaster VTM.
Zach Mouzoun, who arrived on a flight from Geneva about 10 minutes before the first blast, told BFM television that the second, louder explosion brought down ceilings and ruptured pipes.
'It was atrocious. The ceilings collapsed,' he said. 'There was blood everywhere, injured people, bags everywhere.'
'We were walking in the debris. It was a war scene,' he said.
Video taken moments after the explosions showed travellers huddled next to airport check-in counters and lying near luggage and trolleys as dust and the cries of the wounded filled the air. Dazed people stumbled from the scene, some with clothes and shoes blown off.
Anthony Deloos, an airport worker for Swissport, which handles check-in and baggage services, said the first explosion took place near the Swissport counters where customers pay for overweight baggage. He and a colleague said the second blast hit near a Starbucks cafe.
'I jumped into a luggage chute to be safe,' Mr Deloos said.
The subway bombing came after 9am, killing 20 people and wounding more than 100, mayor Yvan Majeur said.
'The Metro was leaving Maelbeek station for Schuman when there was a really loud explosion,' said Alexandre Brans, 32, wiping blood from his face. 'It was panic everywhere. There were a lot of people in the Metro.'
Near the entrance to the station, rescue workers set up a makeshift medical treatment centre in a pub. Dazed and shocked morning commuters streamed from the Metro entrances as police tried to set up a security cordon.
The airport was ordered closed for the rest of the day and chief executive Arnaud Feist said the facility would be closed all of Wednesday and perhaps even longer. More than 200 flights to Brussels were diverted or cancelled, according to flight tracking service Flightradar24.
The Metro also was ordered closed as the city was locked down. By the end of the workday, city officials said residents could begin moving around on the streets of the capital and train stations were reopening. But Peter Mertens, of the Belgian crisis centre, said the threat of more attacks 'is still real and serious'.
At least one and possibly two Kalashnikovs were found in the departure lounge at the airport, according to a European security official in contact with a Belgian police official. It was not immediately clear whether the firearms were used in the attacks.
Travellers fled the airport as quickly as they could. In video shown on France's i-Tele television, men, women and children dashed from the terminal in different directions. Security officers patrolled a hall with blown-out paneling and ceiling panels covering the floor.
Marc Noel, 63, was about to board a Delta flight to Atlanta, to return to his home in Raleigh, North Carolina. A Belgian native, Mr Noel said he was in an airport shop buying magazines when the first blast struck about 50 yards away.
'People were crying, shouting - children. It was a horrible experience,' he said, adding that his decision to shop might have saved his life. 'I would probably have been in that place when the bomb went off.'
'It was chaos'
On the Metro, traveller Evan Lamos tweeted a picture of passengers climbing from his train into the tunnel, saying: 'We are being evacuated from the back of the Metro, between Schuman and Maelbeek. 'Smoke in the tunnel as we evacuate.'The picture was reminiscent of images after the July 7 attacks in London.
Images on social media showed the injured being treated in the street.
As well as the airport, the whole Metro system was closed.
East of England MEP Richard Howitt was in the station shortly before the attack, while Norwich City footballer Dieumerci Mbokani was at Zaventem Airport.
The first news of the attack came when the airport was rocked by a double blast with reports of up to 10 dead and many more injured.
Jef Versele, 40, from Ghent, Belgium, was at the airport when he heard the two explosions.
'I was on my way to check in and two bombs went off - two explosions,' he said.
'I didn't see anything. Everything was coming down. Glassware. It was chaos it was unbelievable. It was the worst thing.'
He added: 'People were running away, there were lots of people on the ground. A lot of people are injured.'
Mr Versele was two or three storeys above the source of the explosion but he said many people around him were hurt.
'The bomb was coming from downstairs. It was going up through the roof. It was big.
'About 15 windows were just blown out from the entrance hall', he added.
The explosion was close to the American Airlines desk.
Images on social media showed shattered windows and smoke rising from an airport building.
People could be seen fleeing in terror in video footage shot from an airport car park.
Footage from inside the building showed a scene of devastation with ceiling tiles strewn across the floor and suitcases abandoned.
Passengers were led onto the tarmac and travellers were urged to stay away from the airport.
Sky News Middle East correspondent Alex Rossi, who was at the airport en route for Tel Aviv, told the channel people were 'dazed and shocked'.
Mr Rossi, who had checked his luggage through the main baggage area and was at a departure gate, said he thought he was 'fairly close' to the explosions but he could not be sure.
He told Sky News as he was being ushered out of the airport along with other passengers: 'We felt the walls of the building rock. Dust came down from the ceiling.'
Airport closed, flights delayed
Brussels Airport, which handles serving 23.5 million passengers per year, told users on Twitter: 'There have been 2 explosions at the airport. Building is being evacuated. Don't come to the airport area.
'Don't come to the airport - airport is being evacuated. Avoid the airport area. Flights have been cancelled.
'All airport operations have been suspended until further notice. Follow the information on our website.
'Passengers that are still located in other areas in the airport are asked to remain calm and wait for further information.'
Prime Minister David Cameron called a meeting of the Government's Cobra emergency committee and tweeted a message of support, saying: 'I am shocked and concerned by the events in Brussels. We will do everything we can to help.'
Foreign secretary Philip Hammond, speaking in Downing Street, said: 'It's an ongoing incident. We are in contact with the Belgian authorities and we will give them every assistance we can.'
Security has been increased at Norwich International Airport and London Gatwick following the explosions in Brussels, an airport spokesman said today.
Heathrow issued a statement which read: 'In the light of events in Brussels airport we are working with the police at Heathrow who are providing a high visibility presence.'
Heathrow warned of disruption to services to and from the Belgian capital and urged passengers to check the status of their flight with their airline.
British Airways said one of its flights from Brussels for Heathrow departed safely at 7.40am but two later services were cancelled.
Two flights making the outbound trip from Heathrow were also cancelled.
The airline said it will make a decision later on flights due to depart this afternoon.
The Foreign and Commonwealth Office (FCO) has updated its travel advice for Brussels.
It issued a statement which read: 'You should stay away from crowded places and avoid public transport at this time.'
British officials in the city have been given the same instructions, the FCO said.
Police numbers have been stepped up at key locations around the UK in the wake of the Brussels attacks.
The country's most senior counter-terrorism officer said the move was a precaution.
Assistant Commissioner Mark Rowley, of the Metropolitan Police, said: 'Our thoughts are with the people of Brussels following this morning's horrific attacks.
'As a precaution forces across the UK have increased policing presence at key locations, including transport hubs, to protect the public and provide reassurance.
'This is not in relation to any specific information or intelligence.'
In London, additional officers have been mobilised, he added.
Paris suspect arrested
Abdeslam, suspected as a planner in the attacks that killed 130 people in Paris, was arrested on Friday after a four-month manhunt, in the same neighbourhood in Brussels where he grew up.
But the Belgian authorities fear he had accomplices while on the run who are still at large and could pose a threat.
Belgian prosecutor Frederic Van Leeuw told reporters at a news conference in Brussels on Monday: '(It's clear) we have a general threat.'
Abdeslam, 26, a French citizen who grew up in Brussels' Molenbeek neighbourhood, slipped through police fingers on several occasions, including the day after the attacks.
He was interviewed three times on Saturday, the day after his capture - once by prosecutors and twice by an investigating judge - and 'wasn't in great shape' because he had been shot in the leg by police during his capture, Mr Van Leeuw said.
Belgian prosecutors appealed to the public on Monday for information about a man who allegedly travelled to Hungary last year with Abdeslam.
Najim Laachraoui, 24, is said to have travelled to Syria in February 2013. He was checked by guards at the Austria-Hungary border while driving in a Mercedes with Abdeslam and one other person.
Laachraoui is said to have rented a house under the name of Soufiane Kayal in the Belgian town of Auvelais that was allegedly used as a safe house. Prosecutors said traces of his DNA were found there.
Abdeslam has a court hearing on Wednesday. France has requested his extradition but Abdeslam's lawyer says his client will fight the request.
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