The heavily fortified U.S. Embassy here was attacked for the fourth time this year today when about 10 men armed with pistols, rifles, submachine guns and at least one grenade launcher opened fire from two or three directions.

An embassy spokesman said there were no injuries to any of the about 100 employes in the building in what he described as the largest and most coordinated attack so far.

Calls to local radio stations said the leftist Popular Liberation Front attacked the embassy to protest U.S. "intervention against the struggle of the Salvadoran people."

A Marine guard on the embassy roof fired one shot, according to the U.S. spokesman, while a nearby contingent of Salvadoran security forces also returned the fire of the assailants. There were unconfirmed reports that one was captured.

Wittnesses and embassy personnel said one group of men opened fire, apparently as a diversion, from the southeast corner of the building while another group fired from the southwest corner nearest acting U.S. Ambassador Frederic Chapin's office.

One Chinese-made rocket-propelled grenade apparently was launched across the street from the main entrance, heavily damaging an unoccupied third floor conference room next door to the ambassador's office. a

The same conference room was damaged by the same type of weapon in an attack last year. Machine-gun fire also hit windows in the ambassador's office, although it is not certain if any penetrated the special glass. Chapin was not in the building at the time of the attack, a spokesman said.

A spokesman said previous attacks have been mostly confined to persons firing from passing pickup trucks. An attack three weeks ago was blamed on rightwing extremists and a more recent shooting was laid to the extreme left.