Suspects arrested during the police raid in Brussels
Police in Belgium say they have arrested 14 people suspected of being members of the al-Qaeda network.
Federal Prosecutor Johan Delmulle said police believed one of those held was planning to carry out a suicide attack.
The suspect had been given the go-ahead for the attack and had already said good-bye to his family, he added.
Correspondents say the authorities appear to have been prompted to act immediately as EU leaders were holding an end-of-year meeting in Brussels.
Security remains tight around the two-day summit in the capital, but Belgian police say it is no tighter than usual for such a gathering.
"It is clear that we have to take the terror threat seriously," Prime Minister Yves Leterme said as he entered the summit on Thursday.
Nearly 250 police officers carried out 16 raids overnight in Brussels and the eastern city of Liege, arrested a total of 14 men and women believed to be linked to al-Qaeda, officials said on Thursday.
Some had recently returned from Pakistan and Afghanistan and at least one might have been planning a suicide attack having talked of recording what was believed to be a martyrdom video, the authorities said. Computers and data storage equipment were also seized.
This information, linked to the fact that the EU summit is being held in Belgium at the moment, left us with no choice but to intervene today
Federal Prosecutor Johan Delmulle
Mr Delmulle said the suspects could have been targeting Pakistan or Afghanistan, "but it can't be ruled out that Belgium or Europe could have been the target".
The man suspected of planning the suicide attack had "received the green light to carry out an operation from which he was not expected to come back", the federal prosecutor quoted investigators as saying.
"He had said goodbye to his loved ones, because he wanted to enter paradise with a clear conscience," he added.
"This information, linked to the fact that the EU summit is being held in Belgium at the moment, left us with no choice but to intervene today."
Mr Delmulle said the raids were linked to a similar sweep last December, when 14 people were arrested by Belgian police on suspicion of planning to free a convicted al-Qaeda member, Nizar Trabelsi, but were subsequently released without charge.
He said an investigation had shown at the time that "a group of people were in Brussels with the task of committing an attack".
"It is now clear to all that we were dealing with a real risk," Belgium's justice and interior ministers later said in a statement.
"It is more than likely that an attack in Brussels has been prevented."
Security services suspect Trabelsi, a 37-year-old Tunisian sentenced to 10 years in prison in 2003 for plotting to bomb a Belgian air base two years earlier, has links with Islamist militants across Europe.
Officials have not released the names of all the people arrested on Thursday, but Belgian media are reporting that they include Malika El Aroud, a Moroccan-born Belgian online writer who was accused by a judge in 2003 of having "utterly extremist ideas".
Ms El Aroud's first husband, Abdessatar Dahmane, died in the suicide bombing in Afghanistan in September 2001 that killed the leader of the anti-Taleban Northern Alliance, Ahmed Shah Massoud.
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