Spanish police arrested 131 suspected Islamic terrorists in 2004, the Interior Ministry said Thursday.

Nearly half of the arrests were in connection with the Madrid train bombings (search ). Forty-two people were arrested as part of an investigation into an Islamic cell that allegedly plotted to blow up the National Court, the nerve center of Spain's anti-terror investigations, said the ministry statement.

Although officials believe most of those involved in on-the-ground planning for the Madrid attacks have been arrested, they warn that other cells are still active and that Spain remains a target.

Interior Minister Jose Antonio Alonso said on Wednesday that Spain remains on high alert against radical Islamic terrorism and the armed Basque separatist group ETA (search ).

During the Christmas holiday, Spain activated a special security plan in an effort to protect the country from a terrorist attack.

Bombings on March 11 ripped through four commuter trains, killing 191 people and wounding 1,600 in Spain's worst terrorist attack. The bombings have been blamed on Islamic militants linked to Al Qaeda (search).

The 62 arrests from those bombings and related incidents helped unravel the plot behind the attacks, the ministry said. That included the planting of a bomb along the high-speed railway line linking Madrid and Seville that did not explode, and the suicides of seven suspects who blew themselves up during a police raid on their apartment.

Twenty people, mostly Moroccans, are in custody in Spain on provisional charges of mass murder or terrorism in the March 11 case. The charges stop short of a formal indictment but suggest the court has strong evidence to convict them.

In total, Spanish security forces arrested 266 people described as "terrorists" last year, the Interior Ministry statement said.

The rest of the detentions were related to ETA, which for years has been the primary focus of law-enforcement officials for the killings of more than 800 people since the late 1960s in its campaign for an independent Basque state.

Among the ETA arrests were top leaders of the armed group, arrested in southern France in October. Sixty-one suspected separatists were arrested outside of Spain, mostly in neighboring France.