Across Europe, heavily armed police flooded into public places and set up checkpoints on normally open borders.
Meanwhile, politicians -- including Britain's David Cameron -- spoke of solidarity. "We must stand together against these appalling terrorists and make sure they can never win."
The beefed up security is to prevent more attacks, but also to reassure nervous travelers.
In France, still on edge after the Paris massacre in November, 1,600 extra police officers were on duty -- especially in railway stations and airports.
But just as important in Europe's response is what you can't see.
As French President Francois Hollande said, "the war against terrorism in Europe must be fought especially using intelligence."
Richard Walton, former counter terrorism chief for Scotland yard, says to do that Europe's security agencies have to share information better.
"There is more to be done about the join up between police and intelligence agencies across Europe, where the borders are more porous, where there is more availability of firearms, and where sometimes the relationship with Muslim communities is not as good as it could be."
After six terrorist attacks on European soil since the beginning of last year alone, there's never been such pressure to get it right.
U.K. officials have been publicly supportive of Belgium, a close neighbor and only two hours away by train. But that didn't stop the Foreign Office from warning Britons not to travel there -- except on truly urgent business.