A bomb exploded Tuesday in the Basque city of Bilbao, seriously injuring a bodyguard assigned to a politician from the Spanish prime minister's party, Basque regional police said.

There was no immediate claim of responsibility, but attention immediately focused on the separatist group ETA, which resumed attacks in August after calling off a cease-fire.

Only hours before the blast, Interior Minister Alfredo Perez Rubalcaba said he had ordered tighter security measures over fears ETA might stage an attack on a major national holiday on Friday.

Tensions are particularly high now because Spanish police arrested 23 senior members of Batasuna, ETA's political wing, on Thursday and 17 of them have been jailed on suspicion of belonging to ETA.

Batasuna has said it interpreted this as a declaration of war against the Basque pro-independence movement and that it was being forced to resort to violence.

ETA called a cease-fire in March 2006, but grew frustrated with a lack of concessions in peace talks with the Spanish government and detonated a huge bomb at Madrid airport in December, killing two people.

It insisted then that the cease-fire still held, but declared it formally over in June and resumed attacks in August, although there have been no fatalities. This appeared to be the first attack to target a specific individual.

Tuesday's bomb went off under the car of a bodyguard assigned to a Socialist member of the city council of the Basque village of Galdakao, near Bilbao. The car was the guard's personal vehicle and he was off-duty at the time of the explosion, a Basque regional police official told The Associated Press. He insisted on anonymity, in line with department policy.

Las Cruces Hospital in Bilbao said the 36-year-old bodyguard was in serious condition with burns to his face and other injuries.

The police official said he did not know if there was a warning call before the blast — ETA often telephones in warnings.

Antonio Rodriguez, a resident of Bilbao, said he was at home watching TV when he heard the blast.

"I looked out the window and I saw this man on the ground rolling himself away from the car," he told the television station CNN+. It broadcast footage of firefighters hosing down the charred, smoking vehicle.

Many public figures and business leaders in the Basque region use bodyguards to protect against ETA attacks.

Before Tuesday's blast, the group had been blamed for three bombings since ending the cease-fire. Only one of them — on Aug. 24 in the Basque city of Durango — caused injuries — two police officers cut by flying glass.