But Interior Ministry officials said Friday's heightened security measures, including increased police patrols at Brussels international airport, rail stations and shopping centres, would remain in place until the start of the New Year.
"They have all been released. But the measures will remain in force until January 2," a spokesman for the Interior Ministry's crisis coordination centre said by telephone.
Under Belgian law, authorities can detain a suspect for up to 24 hours. After that a magistrate must decide whether there are sufficient grounds to hold them further.
Prime Minister Guy Verhofstadt said on Friday there were indications of a possible terrorist attack in preparation after police thwarted what they said was an attempt to free al Qaeda suspect Nizar Trabelsi from jail.
Trabelsi, a Tunisian, was arrested in September 2001 for plotting attacks on U.S. targets in Belgium.
Belgium hosts NATO, the European Union institutions and the offices of a raft of multinational companies.