By DANIEL WOOLLS, Associated Press Writer

MADRID, Spain - A powerful car bomb exploded in Madrid Wednesday after a warning call from the armed Basque separatist group ETA, police said, in the latest of a string of attacks since Spain's prime minister offered talks with the group if it renounces violence.

Eighteen people were slightly injured, said Beatriz Martin, the city's emergency medical department spokeswoman. The explosion occurred around 9:30 a.m. in a working-class district north of the Spanish capital.

Police cordoned off the area where the bomb went off after an anonymous caller to the Basque newspaper Gara, which often serves as a mouthpiece for ETA, said a bomb would explode inside a Renault van.

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