"We are at war. We have been subjected for the last few months in Europe to acts of war," Hollande said in a statement. [Brussels attacked in retaliation for Salah Abdeslam's arrest?]
"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."
The November 13 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.
(With inputs from agencies)