Emergency services in the Belgian capital were scrambled to Zaventem airport at 8am today following two explosions within its departure area.
There have been unconfirmed reports of shots being fired and shouting in Arabic in what has now been confirmed by officials as a suicide attack.
Since the blasts the airport has been closed to flights while all metros have also been suspended after an explosion at Maalbeek metro station within the city.
The station is around 500 metres from the buildings of the European Parliament.
Images appearing on social media show people clambering from a train into a smoke-filled tunnel as the area was evacuated.
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."
Belgium's federal prosecutor said all three explosions were "terrorist attacks".
"There were two explosions in the departure area, one probably caused by a suicide bomber," said Frederic Van Leeuw of the attack on the airport.
It is thought more than 30 people have been killed in the attacks with many more injured. Downing Street has confirmed one Briton is among the casualties.
The terror attacks come days after the arrest of Paris bombing suspect Salah Abdeslam. Since his capture the Belgian capital has been on high alert.
Meanwhile, images on social media showed shattered windows and smoke rising from an airport building.
Video footage shot from an airport car park showed people fleeing in terror while footage from inside the building showed a scene of devastation with ceiling tiles strewn across the floor and suitcases abandoned.
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.
"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.
Passengers were led onto the tarmac and travellers were urged to stay away from the airport.
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 tweeted a message of support, saying: "I am shocked and concerned by the events in Brussels. We will do everything we can to help" as the government release its latest advice for travellers.
The foreign office is urging anyone affected by the incident to follow the instructions of Belgian security authorities and to contact their airline for information about flight arrivals and departures.
People are being asked to remain alert and vigilant in particular in places where there's a high concentration of people.
In the UK, London's Heathrow Airport has also increased security in light of the attacks.
The Belgian Crisis Centre has given the Belgian telephone number 0032 2753 7300 for concerned family/friends or 1171 for general enquiries.