Belgian authorities said 14 people were arrested Friday for allegedly seeking to free an al-Qaida sympathizer imprisoned for planning a terrorist attack on U.S. air base personnel.
They said security was heightened at airports and subway stations.
"Since it could not be excluded that the group had other plans and because of the heightened terror threat this time of year, it was decided no risk should be taken," said Lieve Pellens, spokeswoman for the federal prosecutor's office.
Nizar Trabelsi, a 37-year-old Tunisian who played soccer for several German teams, was sentenced to the maximum 10 years in prison four years ago.
He had admitted planning to drive a car bomb into the canteen at Kleine Brogel, a Belgian air base where U.S. nuclear weapons are believed to be stored. About 100 American military personnel are stationed there.
Trabelsi testified that he intended kill American soldiers, not to detonate the nuclear warheads believed to be at the base.
The prosecutor's office said the 14 suspects were planning to free the terrorist using force.