SANTIAGO, Chile -- A bomb exploded in a Chilean subway station on Monday, injuring at least seven people, the most damaging in a string of bombs planted around the country's capital this year.

Fire Department Commander Ivo Zuvic Garcia said the injuries apparently were caused by fragments in a fire extinguisher placed in a trash bin in front of a food stall at the Escuela Militar subway stop.

Among those hurt in the 2 p.m. blast was a cleaning woman who lost a finger, said Dr. Fernando Zapata of the government's emergency medical service. He said nine were hurt, but other officials put the figure at seven.

Anti-riot police, firefighters and bomb squad officers swarmed across the station, which was closed briefly after the blast

At least 28 bombs have been found across Santiago so far this year -- most planted late at night -- though some have not exploded and none of the others caused any injuries. In many cases, anarchist groups have claimed responsibility, demanding freedom for two anarchists imprisoned in Spain. No one had claimed responsibility for Monday's attack.

Government spokesman Alvaro Elizalde said the bombing "has all the characteristics if a terrorist act that has been carried out to cause harm to innocent persons," and he said the government would invoke an anti-terror enacted during the dictatorship of the 1973-90 dictatorship. It allows lengthy periods of pre-trial detention, longer sentences, interception of communications and masked witnesses.