Belgian authorities on Saturday released 14 suspects detained over an alleged plot to free an Al Qaeda prisoner because of lack of evidence, the Federal Prosecutor's office said.

A court decided there was insufficient evidence to hold them for more than 24 hours, said Lieve Pellens, spokeswoman for the office. She said tightened police anti-terrorism measures triggered by the arrests Friday would remain in place over the holidays.

"We think there is still a threat," Pellens said in a telephone interview.

However she acknowledged that searchs of the suspects homes had found no explosives, weapons or other evidence to persuade the court to keep them in jail.

Police picked up the 14 suspects in a series of early morning raids Friday. Prime Minister Guy Verhofstadt and the prosecutor's office alleged the group was planning to forcibly free Nizar Trabelsi, a 37-year-old Tunisian sentenced to 10 years in prison in 2003 for planning to a drive a car bomb into the cafeteria of a Belgian air base where about 100 American military personnel are stationed.

Authorities tightened security, warning of a heightened threat of attacks despite the arrests. Police stepped up patrols at Brussels airport, subway stations and the downtown Christmas market, which draws large crowds of holiday shoppers.

"Other acts of violence are not to be excluded," warned Verhofstadt.

"According to our investigation there were sufficient indications pointing to a terrorist threat, that is why we did not wait to detain the suspects," Pellens said. "But the suspects have not been formally charged and unfortunately their release does not come as a surprise us."