At least 34 people have been killed and nearly 200 injured as terrorists inflicted a new outrage on a European capital.
Coordinated bomb attacks on the main airport and Metro system in Brussels left a major city in lockdown once again less than five months after Paris was hit by a wave of deadly strikes.
The atrocities on Tuesday, condemned as "blind, violent and cowardly" by Belgian prime minister Charles Michel, came after the arrest last week of terror mastermind Salah Abdeslam, who plotted November's massacre in the French capital.
The terror group Islamic State (IS) has claimed responsibility for the attacks.
Security was being stepped up at major transport hubs around the continent, with British police forces boosting numbers at "key locations" including ports, airports and the rail network.
French prime minister Manuel Valls said: "We are at war. In Europe we have been subjected to acts of war for several months."
A suicide bomber struck first near the American Airlines desk at Brussels airport, with local media reporting that shots were fired amid shouting in Arabic. A US official claimed a suitcase bomb was also detonated.
The explosion on the Metro was near European Union buildings and the US embassy.