Hundreds of police and security officials arrested at least 11 suspected Islamic militants in pre-dawn raids Tuesday in the Spanish African enclave of Ceuta, nabbing them at their homes and while they prayed in local mosques.

The men — including 10 Spaniards and one Moroccan — had recently begun discussing taking concrete action to launch an attack, though they were in the early stages and are not believed to have selected a target, said Interior Minister Alfredo Perez Rubalcaba, who was traveling in Algeria.

"It was an Islamic cell in the process of being formed, so they had no specific targets," Rubalcaba said. "But when we saw that they were moving from fanatic discussion to direct action we moved ahead to detain them."

Among those arrested by officials acting on months of intelligence were two brothers of Hamed Abderrahman Ahmed, a Spaniard detained at the U.S. military prison at Guantanamo Bay, Cuba until 2004, officials said. Ahmed himself was not taken into custody, police said.

Click here to go to FOXNews.com's Europe Center.

Forces sent by boat from the Spanish mainland carried out the operations in neighborhoods all over the city, a police official in Ceuta told The Associated Press. A national police official said that 300 security agents took part in the operation.

Police said they found incriminating documents and audio and video material during the raids, as well as false identity papers, bulletproof vests and a stash of money.

Rubalcaba said that Spanish authorities had received help from Moroccan intelligence agencies.

Ahmed was captured in Afghanistan in late 2001 and held by Pakistani authorities for about four months before being transferred to Guantanamo Bay. He said he had gone to Afghanistan to study at an Islamic school.

He was returned to Spain in February 2004, and eventually found guilty of terror-related charges after an investigation by one of Spain's top anti-terror judges, Baltazar Garzon. But in July, the Supreme Court tossed out the conviction and ordered him freed, saying there was no evidence linking him to al-Qaida.

Tuesday's operation in Ceuta was ordered by Garzon, and the judge is expected to question the detainees in coming days, police said.

Ahmed's brothers were arrested in the working-class, majority Muslim Principe neighborhood of Ceuta, a tiny Spanish enclave on Morocco's northwestern coast, just across the Strait of Gibraltar. In recent years, thousands of would-be migrants have flocked to Ceuta and Melilla, another Spanish town on the African coast, hoping to get a toehold in Europe.

On March 11, 2004, a group of mostly North African Muslim extremists blew up four Madrid commuter trains, killing 191 people and injuring more than 1,500.

Several other plots have been thwarted.

Spanish intelligence agencies have stepped up surveillance of at least 250 Islamic radicals, and keep a close eye on mosques believed to espouse violent views.

They have also launched a series of raids on suspected Islamic militants this year, most recently in November, when authorities in Melilla arrested two alleged members of the Moroccan Islamic Combatant Group, which is believed to have had a hand in a 2003 bomb attack in Casablanca.