"Here we are looking at something different: a well-organised group who were going beyond ideological radicalism to acquiring materials to make explosives and therefore eventually to carry out violent attacks."

Bigger threat
 
Spain's interior ministry has said that it regards Islamist groups, rather than ETA, the armed Basque separatists, as Spain's greatest security threat and has significantly beefed up surveillance of mosques and employed more Arabic translators in the last four years.

The ministry's enhanced measures followed Europe's deadliest Islamist attack in March 2004 in which four Madrid trains were bombed, killing 191 commuters and injuring about 1,800 people.

Twenty-one people have been convicted of involvement in that attack.

In September 2004, 11 Pakistanis were arrested in Barcelona, accused of planning attacks in the port city.

All were cleared, but three were jailed last year after raising money destined for "terror" groups, while two others were found guilty of forging documents.

Since the September 11, 2001 attacks in New York and Washington, Spanish police have arrested hundreds of suspects, many in connection with the Madrid attack.

In recent years Spanish police have also focused on cells suspected of recruiting mujahedeen fighters and suicide bombers, or of collecting money to finance al-Qaeda-linked groups abroad.