SANTIAGO, Chile (AP) _ Chile's main leftist urban guerrilla group denied responsibility Friday for a bomb blast at a military pensioners' medical clinic that killed a 64-year- old woman and wounded 20 people.

Government officials said they suspected leftist extremists were to blame for the Wednesday bombing, without specifying one organization.

''It had nothing to do with us,'' a spokesman for the Manuel Rodriguez Patriotic Front told The Associated Press by telephone. ''It doesn't coincide with our method of working or with our objectives. Nor is it the kind of target we act against.''

He identified himself as Jorge Salas. A man using the same name frequently has called news agencies to give information about the pro-Communist group's actions.

The bombing occurred at a medical clinic used by retired military personnel and their relatives in downtown Valparaiso, a port city 85 miles northeast of Santiago.

The Front spokesman called to deny allegations Thursday by Attorney General Ambrosio Rodriguez that the Front belonged to an international guerrilla network and that it formed a pact with another, less active extremist group in Chile, the Leftist Revolutionary Movement.

''We have no pact ... with any other group in any country in the world, particularly in Latin America,'' he said.

Asked about local press reports that the Front might be planning another assassination attempt against President Augusto Pinochet, the spokesman said: ''Well, Pinochet has been a permanent target for us. He has never stopped being one.''

In September 1986, the Front ambushed Pinochet's motorcade, killing five of his bodyguards. Pinochet escaped with a scratched hand.