MADRID—The operation was ordered by Spain's top anti-terrorist judge Baltazar Garzon and was still ongoing, the source told AFP.

Six arrests took place in Barcelona, although all of those concerned had travelled from different parts of Spain, said another source.

They are suspected of financing terrorist activities by carrying out thefts and sending the money they raised from their criminal activities to Pakistan, the judicial source added.

Neither source provided the nationality of those arrested.

In March 2004, early morning bomb attacks on three packed commuter trains in Madrid killed 191 people in Europe’s worst Islamic-linked terror attack.

Islamic extremists claimed responsibility for the attack, which they said they had carried out in the name of Al-Qaeda in response to the presence of Spanish troops in Iraq at the time.

Garzon has warned that Spain faced a "very high risk" of suffering another Islamist attack.

Spanish police have carried out several operations in recent years against suspected Islamic extremists.

Police detained 15 suspected Islamic militants, mostly from Pakistan, in January 2008 in Barcelona who the authorities feared were plotting an attack in the city, which has a large population of Pakistani immigrants.

AFP/Expatica