Belgian authorities say anti-terrorist police are holding a suspect over the suicide bomb attacks in the Moroccan city of Casablanca last year.
Five simultaneous attacks rocked Casablanca
Police raided around 20 houses in Brussels, where the unnamed suspect was detained, and other Belgian cities.
They said they were investigating North Africans who had received paramilitary training in Afghanistan.
About 45 people, including 12 suicide bombers, were killed in the attacks in the Moroccan city in May.
Spanish police are believed to be probing links between those attacks and the train bombings in Madrid last week, says the BBC's John Moylan in Brussels.
Hundreds of people have already been arrested in connection with the Casablanca bombings, and trials have resulted in more than 10 death sentences.
"One of the people detained is... the subject of an international warrant delivered by the Moroccan authorities following the attack in Casablanca [in May 2003]," an official statement said, carried by the Associated Press news agency.
The suspect is believed to be linked to a group called the Moroccan Islamic Combatants Group which has been under investigation by Belgian police
and intelligence services, the statement from the Federal Prosecutors' Office
The inquiries have revealed "serious evidence" pointing
to the presence in Belgium of a group of North Africans
linked to that group who have received paramilitary
training in Afghanistan, the statement said.
Some are illegally resident in Belgium, it added.