There are no immediate reports of casualties, although a large cloud of smoke produced by the blast led to traffic being blocked off, Spanish news agency Efe reported.

Smoke was pouring from ventilation shafts in the car park, and Spanish police were proceeding with caution because of fears of a second car bomb nearby, Reuters news agency reported.

Police said a Basque newspaper had alerted them before the explosion, which had given them time to cordon of the vicinity and evacuate the car park.

Warning

Authorities are assessing the damage, although it does not appear to be serious.

The newspaper reportedly received a call from the Basque separatist group ETA about 40 minutes before the bomb went off.

ETA has killed more than 800 people and injured more than 2,300 since 1968 in its campaign for an independent Basque homeland in north Spain and south-west France.

Its most recent attack was in September, when a booby-trapped bomb just outside the northern town of San Sebastian went off, killing a policeman.

And in June this year a blast in Santander blamed by Spanish police on ETA slightly injured a policeman.

The Spanish Government and courts have cracked down on Basque nationalist party Batasuna, accused of being ETA's political wing, and has taken steps to have the party outlawed.