Many more people were injured after terrorist explosions on the main airport and Metro system in the Belgian capital.
The coordinated attacks came as the city was on a high level of alert following the arrest of Paris atrocity suspect Salah Abdeslam in the city last week.
There were unconfirmed reports of shots being fired and shouting in Arabic at the airport in what Belgian media said was a suicide attack.
At least one explosion hit the Metro system near a station close to European Union buildings.
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."
Bishop Athenagoras Peckstadt, chaplain at Brussels Airport, told Premier: "I knew very soon after the first explosion that something had happened.
"I tried to go to the airport but they told us no can enter into the airport. The atmosphere in Belgium is very bad. More people tried to call me to ask if I'm OK. So, all the people live with this very bad moment."
Responding to events in Brussels this morning, the Archbishop of Canterbury said: "In the great Holy Week of Christian prayer and mercy, the Brussels attacks shock all those who seek peace and justice through the terrible cruelty and utter separation from all that is of God.
"Once again we see the contrast between the vain efforts to terrify through indiscriminate murder, and the call of God to be those who show mercy, who seek peace and pursue it.
"Let us at every service this week pray for those caught up in the traumatic events at the airport and in the City of Brussels."
Revd Dr Andrew Gardener, from St Andrew's Church of Scotland in Brussels, told Premier: "The airport is my second home sometimes.
"This is a place that many of the people in my congregation know very well, and it will raise huge amounts of fear among the ex-pat community as a whole.
"Easter as you know is a prime point in celebration in the Christian calendar and our Easter celebrations will be sobering this Sunday.
Revd Dr Andrew also said churches are afraid to open their doors, in case terrorists attack them too: "There's an element of fear [that] people who you wouldn't want to know that you're gathering would turn up and target those of us who are gathered."
Simon Barrett, Christian a terrorism expert at International Intelligence Media Analysis told Premier: "When I woke up to the news that the airport has been attacked, I thought there could another one because that's the style of ISIS.
"They like multiple-site attacks, primarily because it gets more attention but also then diverts the security services to other places across the city and just puts the whole [place] into chaos.
"The [security] levels will go up in Britain. They're already very high anyway. It'll mean the police will be a lot more vigilant and people will be a little bit concerned that something could happen here."
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.
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 London Gatwick following the explosions in Brussels.
An airport spokesman said the safety of passengers and staff was the "absolute priority".
He added: "As a result of the terrible incidents in Brussels we have increased our security presence and patrols around the airport."
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."
Listen to Revd Dr Andrew Gardener speaking to Premier's Aaron James:
This page is being updated.
