People have been evacuated from the Brussels Metro network following an explosion at Maalbeek station.
Video footage shows people descending from the carriages of a train between Arts-Lois and Maalbeek stations.
The entire public transport system in the Belgian capital has been shut down following a series of explosions. It is feared that at least 20 people have died and many more have been injured.
The terror alert in Belgium has been raised to its highest level.
Shortly after 8am on Tuesday, two explosions were heard in the departure hall of Brussels Airport, one near the American Airlines check-in area and another near the Brussels Airline desk.
Video footage showed people fleeing as grey smoke was seen rising from the terminal building.
Shortly after, another explosion took place at Maalbeek Metro station.
The cause of the blasts is not yet known.
The Belgian federal prosecutor said all three explosions were "terrorist attacks" and that at least one at the airport was probably caused by a suicide bomber.
Transport operator STIB told Belgian broadcaster RTFB that the explosion in the Metro killed 15 people and injured 55, 10 critically. And Belgian media reported that 13 people had been killed at the airport.
All flights at Brussels Airport have been cancelled for the rest of the day and arriving planes have been diverted. Security has been increased at airports across Europe.
Zach Mouzoun, who arrived at the airport before the first blast, said: “It was atrocious. The ceiling collapsed. There was blood everywhere, injured people, bags everywhere.”
He added: “We were walking in the debris. It was a war scene.”
Charles Michel, the Belgian Prime Minister, said: "What we feared has happened, we were hit by blind attacks."
The blasts come after Salah Abdeslam, a suspect in the deadly Paris terror attacks, was captured in Brussels on Friday. On 13 November, 130 people lost their lives in coordinated attacks in the French capital.
Mr Michel said there was no information on whether the attacks related to Abdeslam's arrest.
Additional reporting by agencies
