BRUSSELS, Belgium -- Belgian authorities detained 14 people they described as Islamic extremists yesterday, saying they had uncovered a plot to use explosives to free an al-Qaida sympathizer jailed for planning to attack a U.S. air base.
The Belgian prime minister, Guy Verhofstadt, said in a statement, "Other acts of violence are not ruled out."
The authorities put the capital, Brussels, on a high state of alert, increasing security at main train stations, the airport and major public places where people were gathering to do their Christmas shopping.
The arrests came after the police raided 15 locations, most of them in Brussels, seizing explosives and arms.
Those detained were suspected of planning to try to break into a prison to free Nizar Trabelsi, a Tunisian former pro soccer player who was arrested days after the Sept. 11, 2001, terrorist attacks in the United States, in connection with a plot to drive a car bomb into an American air base in northeastern Belgium. Trabelsi was convicted in 2003 and sentenced to 10 years in prison.
The authorities did not offer any details about their suspicions or name the prison where Trabelsi was being held. A spokesman for the Interior Ministry, Peter Mertens, said the convict was "moved regularly."
Europe is already on a state of alert because of the Christmas holidays and the Algerian bombings last week, which killed dozens in the capital, Algiers. France and Belgium share concerns of terrorist threats from extremists among their Muslim populations.
On Thursday, French police said they were holding five men believed to be members of a logistical support cell for al-Qaida in the Islamic Maghreb. That group is a long-standing terrorist group, previously called the Salafist Group for Preaching and Combat, which changed its name after affiliating with Osama bin Laden's network this year.
The five men were part of a group of eight people - six French citizens, an Algerian and a Tunisian - who were detained early Tuesday in Paris, its outlying districts and in the Rouen region of northwestern France, the news agency said, although three men were released Wednesday.
The newspaper Le Figaro described the arrest of the eight as "one of the biggest in 2007." It capped months of investigation, the agency reported.
A lawyer who represented Trabelsi at his trial but who no longer acts for him, Didier de Quevy, said he did not know whether his former client was aware of any plot to free him. "Anything is possible," said De Quevy. "He is being held in harsh conditions, and he could have acted out of desperation, but it does not correspond to the image of the person I knew, who was motivated by strong religious convictions."
De Quevy said he had not spoken to Trabelsi for about nine months.
In November 2005, Belgian police arrested 14 suspects in a series of raids aimed at breaking a terrorist network that the authorities said was involved in attacks on American targets in Iraq, including a suicide bombing by a Belgian woman in Baghdad in 2005.
At the time, Belgian police said the group was recruiting volunteers across Europe to assist al-Qaida in Mesopotamia, an extremist group of Sunni Iraqis that is a major force in the Iraq insurgency. American intelligence says it is foreign-led.