A bomb placed under the car of a U.S. Embassy employe exploded at the gates of the embassy complex in Lisbon today shortly after it was discovered in a routine security check, embassy officials and police said. No one was hurt.

Embassy spokesman Dan Traub said Portuguese security guards operating a checkpoint at the gate ordered the area cleared when they spotted a suspicious device under the employe's car. Shortly thereafter a blast ripped through the car, he said.

Traub said people in the vicinity were lucky to escape injury in the explosion that destroyed the car but caused no other damage. No warning was given before the explosion and so far no group has said it was responsible, according to officials.

It was the third time the U.S. Embassy here has been the target of terrorist attacks since it was opened in July 1983. A leftist guerrilla group said it was behind two attempts to shell the building with mortars in November 1984.

The same group, calling itself the Popular Forces of April 25, also asserted responsibility for slaying the director of Portugal's prison services outside his Lisbon home on Saturday. The group said the shooting was in retaliation for the allegedly poor conditions under which more than 70 suspected guerrillas are being held.

U.S. Embassy public affairs officer Steve Chaplin said the American whose vehicle was boobytrapped was driving the car into the embassy grounds when the bomb was discovered. Chaplin identified the man only as an embassy employe who was not a high-ranking official.

Chaplin said police experts had not yet determined the type or exact size of the bomb. The U.S. Embassy, newly built as part of a worldwide program to strengthen security at U.S. government installations, stands on a high rise surrounded by a low wall and separated from nearby streets by extensive grounds.

The checkpoint where the bomb was discovered is at the only entrance to the complex more than 100 yards from the main building.

The guerrilla group is named after the date of Portugal's 1974 revolution. It has asserted responsibility for previous attacks on the embassy and is held responsible for a series of bombings, shootings and bank holdups in which more than 12 people have been killed since 1980.

The group said it fired four mortar shells that hit in and around the embassy grounds in November 1984 causing slight damage but no injuries. Three weeks earlier the group said a makeshift rocket launcher found near the embassy had been designed to fire mortars at the building but had failed to function as planned.