Some 500 friends and supporters memorialized slain Philippine opposition leader Benigno (Ninoy) Aquino yesterday as a martyred victim of the "institutional violence" they said has been bred by the government of Philippine President Ferdinand E. Marcos.

The memorial mass at Georgetown University Chapel came less than 24 hours after three unidentified youths hurled firebombs into the Philippine Embassy at 1617 Massachusetts Ave. NW, shattering the chancery's front door and briefly setting the lobby afire.

No one was injured in the blaze, which embassy information officer Ray Naval said caused only "negligible damage."

D.C. police, FBI and Secret Service spokesmen said they had no immediate leads on the firebombing. United Press International, however, quoted an embassy source as stating the incident occurred about two hours after a threatening anonymous telephone call from someone with a Philippine accent.

"Why are you not practicing democracy in the Philippines? We'll get even," the caller reportedly said.

The embassy has received several bomb threats since Aquino was gunned down last Sunday moments after stepping off a plane in Manila from three years of self-imposed exile in the United States.

At yesterday's 90-minute mass, a procession of speakers portrayed Aquino, a political opponent of Marcos, as a man of faith and compassion who returned to his homeland, despite repeated threats against his life, to arouse his countrymen to the need for nonviolence and Democratic reforms.

" 'Why are my countrymen so apathetic?' he asked," said former Philippine foreign minister Raul S. Manglapus, now of McLean. " 'I will go home again and stir things up . . . even if they shoot me.' Well, they shot him, and at this moment there is no apathy, at least not in Manila and greater Luzon."

Manglapus added, "Ninoy's spirit is stronger and will be even stronger than when he was alive." He compared the slain leader to Jose Rizal, the 19th Century Filipino national hero who sought nonviolent reform only to be executed by the Spanish and spark the national revolution he had not desired.

"What will Ninoy's death provoke?" Manglapus asked. "We ask here today, 'Who pulled the trigger on Ninoy Aquino?' "

With Biblical readings, personal statements and folk songs in Tagalog, the native language of the Philippines, a procession of Filipino students urged their countrymen to vigilance but not violence and announced a candlelight vigil in front of the White House tonight as funeral services for Aquino are conducted in the Philippines.

At the Philippine Embassy, meanwhile, Naval said repairs were already underway from the firebombing, which he said occurred about 10:10 p.m. Saturday. He said that a witness told police he saw three young men throw what appeared to be homemade molotov cocktails against the door of the embassy.

Naval said at least two and possibly three bombs were thrown before the men fled north on 16th Street. The witness, who had been driving west on Massachusetts Avenue when the incident occurred, gave chase, Naval said, but lost the youths in the darkness.

The men appeared as slender and well dressed, one with a bandana around his neck, Naval said, and were described as "looking like Latinos." He said three embassy employes were in the building at the time of the incident but were nowhere near the lobby.