Police on Wednesday arrested three Moroccans they believe were part of a radical Islamic cell looking to buy explosives in a central European country for a terrorist attack in Spain, the Interior Ministry said.

Majid Bakkali, Mohamed Douha and Abdelkader Farhaoui were arrested in the northeastern towns of Sant Andreu de la Barca and Mollet de Valles before dawn, the ministry said in a statement.

The three were part of a cell "that was engaged in different activities leading to the purchase of explosives, with the aim of committing terrorist attacks in our country," the ministry said.

The arrests came within hours of Prime Minister Jose Luis Rodriguez Zapatero's (search) announcement that, as of midnight Wednesday, Spain would activate a special security plan to cover the Christmas holiday period to try to protect the country from a terrorist attack. The plan lasts through Jan. 8.

Islamic militants are blamed for the Madrid bombings (search) of March 11 that killed 191 people and injured 1,800. Dozens have been jailed in the attacks and with alleged plans to carry out more bombings.

The ministry said police had begun watching the three Moroccans in September when they made contact with a suspected intermediary in an unspecified central European country where they planned to buy explosives.

Police searched the Moroccans' homes, a phone shop and an Islamic butcher's shop, the ministry added, confiscating documents and computer material.

Separately, a judge Tuesday charged a Moroccan suspect with terrorism and mass killings for allegedly helping plan the March 11 bombings.

Judge Juan del Olmo said Hasan al-Haski, who was arrested in the Canary Islands last week, is a suspected prominent member of the Moroccan Islamic Combatant Group (search), an Islamic organization with alleged ties to Al Qaeda that was blamed for the 2003 Casablanca attacks that killed 45 people, including 12 bombers.

Another Moroccan arrested along with al-Haski, Abdallah Mourib, was jailed on terrorism charges, while two others were released.