MANILA, AUG. 21 -- As President Corazon Aquino and her children gathered today at her slain husband's grave, 5,000 leftist demonstrators commemorated the opposition leader's assassination four years ago with a bitter protest rally against the policies of his widow's administration.

The protesters, led by leftist labor, peasant and student leaders, accused Aquino of "betraying the legacy" of her murdered husband, former senator Benigno (Ninoy) Aquino. The marchers demanded a rollback in an oil price increase approved last week by the president.

The angry demonstration outside the presidential palace was the latest sign of growing dismay with Aquino's government. Her leftist critics have complained that wages remain too low and that promises of sweeping land reform have not been fulfilled. Meanwhile, critics on the right have charged that Aquino has been feckless in dealing with a spreading Communist insurgency.

Benigno Aquino, political arch-rival of deposed president Ferdinand Marcos, was assassinated Aug. 21, 1983, while in the custody of government soldiers who had escorted him off an airliner at Manila airport. He was returning from years of self-exile in the United States.

The protesters today gathered in a downtown plaza, then marched several blocks to barricades that police had erected 500 yards from the palace.

Several speakers accused President Aquino of allowing the U.S. government and foreign businesses to dominate the country. Before dispersing, the crowd burned effigies of President Aquino and Uncle Sam. Earlier in the day, 2,000 people packed the suburban Santo Domingo Church for a mass attended by Aquino and other top government officials.

In remarks at the service, President Aquino said, "Without Ninoy, I would not be here, president of this great and proud country.

Benigno Aquino's death sparked huge street demonstrations and gave birth to a broad, moderate anti-Marcos movement. The protests led to a civilian-backed military revolt that forced Marcos to flee the Philippines in February 1986.

Benigno Aquino became the rallying symbol of Filipinos opposed to Marcos' rule.

The 1983 assassination generated in Filipinos a vast reservoir of sympathy for Corazon Aquino that has served as a powerful political advantage in her first 18 months in office.

But as Filipinos gathered today to honor Benigno Aquino, there were signs that some are beginning to question the president's stewardship of the "Aquino legacy."

Even the Philippine Daily Inquirer, a staunchly pro-Aquino daily, sounded a note of disillusionment in an editorial entitled "Ninoy's murder in perspective."

"His vision of sweeping social change, which Aquino's followers promised as they rose to power, has remained unfulfilled," the paper said. "Millions of those same Filipinos who the slain senator said were worthy of his martyrdom are still captives of poverty, victims of either official neglect or elite-directed policy."