Belgium officials say they have released 14 suspects detained Friday in an alleged plot to free an al-Qaida prisoner, because of a lack of evidence.
Authorities say the investigation continues.
Heightened security measures remain in effect across Belgium, and authorities say the security alert will last at least through New Year's Day.
Security was tightened at airports, rail stations and other public places after police made the arrests Friday. Belgian government officials say the suspects were planning to use explosives to stage a prison break for an al-Qaida loyalist who planned to attack a Belgian air base that houses American military personnel.
Belgian authorities say they are concerned about possible acts of violence during the Christmas and yearend holiday period.
The U.S. Embassy in Brussels advised American citizens Friday that there is a heightened risk of terrorist attack in Belgium.
Police say the new security precautions are intended as a precaution against unspecified possible acts of violence.
In the raids early Friday, police said they seized members of a group planning to free Nizar Trabelsi, a former professional footballer. Currently serving a 10-year prison sentence, Trabelsi is a Tunisian national who was arrested two days after the September 11th terror attacks in the United States in 2001.
Some information for this report was provided by AFP, AP and Reuters.