The series of raids across the continent highlighted fears about young Europeans travelling to fight war with the Islamic State and other extremist groups in the Middle East, and then returning to launch attacks on western targets.
In Belgium, officials said they had averted "imminent" large-scale attacks on police targets after raiding a terror cell in the eastern town of Verviers, near the German border, whose members had recently come back from Syria.
Prime Minister Charles Michel raised Belgium's terror alert to its second highest level, security was tightened and Jewish schools in the port city of Antwerp closed today due to fears of further trouble.
The raid and a series of related search operations across Belgium were now "over" but authorities were now seeking to "exploit the information" they had obtained, Foreign Minister Didier Reynders said.
"The threat was to the police forces," he said of the planned attacks.