Tuesday's attacks in Brussels that killed at least 28 people struck at "the whole of Europe", French President Francois Hollande said.
"Through the attacks in Brussels, the whole of Europe has been hit," Hollande said in a statement, urging the continent to take "vital steps in the face of the seriousness of the threat.
"France which was itself attacked in January and November last year is fully engaged in that. France will implacably continue the fight against terrorism both on the international level and at home."
Belgian Prime Minister Charles Michel called the attacks in Brussels “a tragic moment”, and said he feared additional acts of terror against Belgium.
“What we feared has happened,” said Michel, “we were hit by blind attacks.”
“We realize we face a tragic moment. We have to be calm and show solidarity.”
One of the attacks struck a subway station just a few hundred yards away from the European Union's seat of government.
A federal prosecutor labelled the three bombings “terrorist attacks”, and confirmed reports that at least one of the explosions appeared to have been a suicide bomber.
Last Friday Belgian police captured a suspect from the November 13th Paris attacks, Salah Abdeslam. Some analysts have suggested that Tuesday’s attacks may have been in retaliation for the arrest.
The November attacks in Paris claimed 130 lives 10 months after attacks on the satirical newspaper Charlie Hebdo and a Jewish supermarket left 17 dead.
French Interior Minister Bernard Cazeneuve earlier announced that Paris was deploying 1,600 additional police to border crossings and air, sea and rail infrastructure after the Brussels attacks.