Two dozen people were killed on Tuesday in terrorist attacks that targeted the airport and a subway station in Brussels, with the Belgian capital under lockdown as authorities rushed to secure other sites.
Two explosions that ripped through the departure hall at the Brussels airport left 11 people dead and 81 injured, the Belga news agency quoted Belgian Health Minister Maggie De Block as saying.
The royal prosecutor called the airport blasts, which occurred around 8 am (0700 GMT), a suicide bombing.
About an hour later - just as many commuters were headed to work - a second explosion was reported in the Brussels subway station of Maelbeek, which is located near EU institutions.
That attack killed 15 people and left 10 seriously injured, the STIB public transport operator told Belgian media. Another 45 people suffered minor injuries, it added.
The federal prosecutor's office said in a statement that the three explosions "were terrorist attacks" and that a terrorism investigation has been launched.
Belgian authorities are working to secure "other sites" that are of concern, Prime Minister Charles Michel said, adding that military reinforcements have been deployed and controls are being carried out at Belgium's borders.
German police were tightening measures at the country's border with Belgium. A spokesman declined to give further details but said the move is part of a series of security measures mounted by police at Germany's borders, airports and train stations.
Michel called the attacks "blind, violent and cowardly."
"In this moment of tragedy, in this dark moment for our country, more than ever I want to call on everyone to display calm, but also solidarity. We are confronted with an ordeal, a difficult ordeal and we have to face this ordeal by being united," Michel said.
The chief of the NATO military alliance, whose headquarters are about 5 kilometres away from the Brussels airport, said the killings were "an attack on our values and on our open societies."
"Terrorism will not defeat democracy and take away our freedoms," NATO Secretary General Jens Stoltenberg said in a statement.
A spokesman for German Chancellor Angela Merkel called on Twitter for "solidarity with the victims" and "determination towards the terrorists."
Belgium raised its terror alert level to 4 after the Brussels airport attack, indicating a "serious and imminent" threat. The alert level at the NATO headquarters has also been increased, Stoltenberg said.
The country had already been on high alert since the arrest in Brussels last week of Salah Abdeslam, a key suspect in the Paris terrorist attacks. Many of the perpetrators of those killings in November had links to the Belgian capital.
In the French capital, President Francois Hollande held an emergency meeting with top ministers, including Prime Minister Manuel Valls and Defence Minister Jean-Yves Le Drian.
"We are faced with a global threat," Hollande said. "The war against terrorism must be conducted across Europe."
Some 1,600 additional security forces have been deployed to secure France's airports and train stations after the Brussels attacks, French Interior Minister Bernard Cazeneuve said.
In the Belgian capital, the entire public transport network was shut down Tuesday, while the city's tunnels were closed to car traffic.
Train traffic across Europe was disrupted following the attacks, with the Eurostar and Thalys high-speed operators saying that none of their trains were running to or from Brussels.
Thalys said one of its trains departing from the Paris station Gare du Nord was redirected to the French city of Lille, and trains have ceased departing from stations in Germany and the Netherlands.
International institutions in Brussels sent staff home. Authorities called on people to stay where they were and not to make phone calls, with the mobile network overburdened.
The airport, located about 15 kilometres from the Brussels centre, cancelled all flights after the attack. Train traffic to the airport was also halted, the SNCB rail company wrote on Twitter.
Pictures posted on social media showed a blown-out window front, debris that appeared to come from a collapsed roof and bloodied people inside and outside the airport. Smoke was also seen rising from the building.
The explosions at the airport took place one after the other, the Belga news agency wrote, quoting eyewitnesses who said that they had heard shots and calls in Arabic beforehand.
Weapons were found in the Brussels airport, Belga reported in the afternoon. A Kalashnikov rifle was discovered in the departure hall, according to the RTBF broadcaster.