Belgian police detained 14 suspected Islamic extremists, and the government said they were plotting a jailbreak to free an al-Qaida prisoner convicted of planning to attack US military personnel.
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," said Prime Minister Guy Verhofstadt.
The US Embassy warned Americans that "there is currently a heightened risk of terrorist attack in Brussels," although it said it had no indication of any specific targets.
In a series of overnight raids around the country, police picked up 14 suspects and seized arms and explosives. The prime minister and prosecutor's office alleged the detained were planning to use the weapons to 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.
The Kleine Brogel base was reported to house US nuclear weapons, although that has not been confirmed by US or Belgian authorities. Security services in several European nations suspect Trabelsi, who trained with al-Qaida in Afghanistan, had links with extremists in Britain, France and elsewhere in Europe.
"Trabelsi would be helped by a group of people driven by an extremist vision of Islam," the Federal Prosecutors' Office said in a statement on the arrests.
Details about the identities of the suspects were not immediately released, but the RTL-TVI television network said one of those detained was Malika El Aroud, the Moroccan-born widow of one of the suicide bombers who killed Afghan anti-Taliban leader Ahmed Shah Massood two days before the September 11, 2001, attacks on the US.
El Aroud, a Belgian resident, was acquitted in a Brussels court of involvement in Massood's killing in 2003. In June, she was convicted in Switzerland for supporting a criminal organization by running Web sites that posted statements from al-Qaida-linked groups and showed executions.
She received a six-month prison sentence suspended for three years, which would keep her from going to jail unless she commits another punishable offense during the time.
Within hours of the arrests, police were out checking for suspicious packages and bags in the capital's landmark Grand Place, where tourists and local shoppers mingled beneath a huge Christmas tree. The Interior Ministry urged citizens to report anything suspicious.
"If a group with an extremist view of Islam was ready to use arms and explosives to release Mr. Trabelsi, there is no guarantee that they would not use them for other ends," said Lieve Pellens, spokeswoman for the Federal ProsecutorsÕ office.
The extra security measures were expected to last until at least January 2.
Prison Inspector Claude Spinoit told RTL-TVI that he had spoken to Trabelsi on Thursday. He said the former professional soccer player was being held under tight security in Nivelles prison, 22 miles (35 kilometers) south of Brussels. Spinoit described Trabelsi as "calm" and said he had asked for isolation in order to pursue religious contemplation.
Trabelsi came to Europe in 1989 for a tryout with the German soccer team Fortuna Duesseldorf. He got a contract but was soon let go. Over the next few years, he bounced from team to team in the minor leagues, acquiring a cocaine habit and a lengthy criminal record.
Eventually, he made his way to al-Qaida training camps in Afghanistan, where evidence presented at his trial showed he placed himself on a "list of martyrs" ready to commit suicide attacks.
Trabelsi has admitted planning to kill U.S. soldiers. He said he met al-Qaida chief Osama bin Laden in Afghanistan and asked to become a suicide bomber. He was arrested in Brussels two days after the September 11 attacks and police later linked him to the discovery of raw materials for a huge bomb in the back of a Brussels restaurant.
"Trabelsi is an important figure for armed Islamic circles. He is a highly symbolic figure who has met Osama bin Laden," said Claude Moniquet, president of the European Strategic Intelligence and Security Center, a Brussels-based think tank specializing in terrorism issues.
Moniquet, who has contacts with the Belgian security forces, said he understood those arrested appeared to be part of a "classic" type of extremist cell in Europe and was made up of local men of North African origin. "It appeared to be a serious plot," he said.
The US Embassy issued its alert to Americans living or traveling in Belgium, advising them to maintain a high level of vigilance, especially in crowded public places. The statement said it had "no information to indicate that US citizens or facilities are an intended target."
Moniquet said the location of NATO and European Union headquarters in Brussels made the Belgian capital a potential target for terrorists, but the danger was similar in other European capitals. The threat level is always very high, he said.
Belgian authorities have faced calls to tighten prison security after a series of high-profile recent escapes. In April, two men hijacked a helicopter at gunpoint and forced the pilot to pluck a French prisoner from the exercise yard at Lantin prison. The prisoner, who was in pretrial detention on charges of fraud and theft, was later re-arrested in Italy. (AP)