A bomb planted under a seat of a movie theater exploded here today, killing four persons and wounding 28 in a crowded section of mainly Moslem West Beirut.

A spokesman for a group calling itself the Front for Liberating Lebanon From Foreigners claimed responsibility for the explosion in telephone calls to two news agencies.

The same group claimed credit for three other explosions on Thursday and Friday. The latest blast brought the death toll to 32 and the number of injured to at least 110, security sources said.

"This operation is to prove to the Palestine Liberation Organization that we are not an imaginary organization," the caller told Reuter news agency.

The cinema, in a small, two-story building, was featuring a karate film entitled "The Dangerous Man." Ambulances rushed to the theater to rescue the wounded trapped inside as firemen battled a blaze caused by the explosive charge.

Angry young men brandishing AK47s fired warning shots into the air as crowds gathered to observe rescue operations.

Ambulances mounted with loudspeakers sped through the area appealing to residents to make blood donations to nearby hospitals.

Syrian troops of the Arab Deterrent Force blocked off traffic to facilitate the transportation of the victims to nearby clinics.

Cameramen who were allowed to film inside the movie hall said blood covered broken chairs scattered across the floor. Television showed several teen-agers wrapped in bloodied sheets on stretchers in the theater.

The movie theater lies barely half a mile west of the Museum Road, a crossing point between West Beirut and Christian East Beirut that was opened to civilian traffic only a week ago after intense diplomatic efforts by an Arab mediation committee.

It is also a stone's throw from the residence of the late French ambassador, Louis Delamare, who was killed Sept. 4 by gunmen who reportedly tried to kidnap him.

Museum Road is one of several intercity links along the explosive Green Line.