PORT-AU-PRINCE, Haiti (AP) _ A Western ambassador and a high-ranking army source said today that Haiti's ruler, Lt. Gen. Prosper Avril, will step down in the face of violent anti- government demonstrations.

Avril was to resign today and hand over power to the army chief who will preside over a transition to civilian rule, the sources said on condition of anonymity. The Western ambassador said Avril will relinquish power to Maj. Gen. Herard Abraham, the army's commander-in-chief.

Abraham, in turn, will have 72 hours to hand over the government to a civilian council headed by a Supreme Court justice, the ambassador said.

The Army High Command source said Avril would remain in Haiti after stepping down and a 48-hour curfew would be imposed to prevent disorder.

The prediction came after five days of anti-government demonstrations and calls by an opposition coalition that Avril relinquish power.

The army source said the main problem facing the High Command was how to handle possible resistance among the 1,000-member Presidential Guard, many of whom are hostile to the idea of democratic government.

A palace source said, however, that the guards would not resist the transfer of power. The source also spoke on condition of anonymity.

If Avril steps down, it will be the fifth change in leadership in Haiti since 1986, when President Jean-Claude Duvalier fled the impoverished country during widespread protests, ending a 29-year family dictatorship.

The idea of setting up a civilian council headed by the Supreme Court's vice president, Gabriel Volcy, was proposed last week by the Unity Assembly, an umbrella group of 11 opposition political parties and a civic organization. The council is to prepare for legitimate elections.

On Friday night, the Unity Assembly issued a statement calling for an ''unlimited total paralysis of the country'' if Avril didn't resign by Monday.

Also Friday, troops fired on 250 student demonstrators in Port-au-Prince, and three protesters were reported wounded.

State television said Avril met with his Army High Command and with church leaders about ''bringing peace'' to Haiti.

A U.S. Embassy diplomat said the United States was not involved in negotiations with Avril but added without elaboration that the talks were expected to be successful.

''We are keeping in touch with them but we are in no way involved with negotiations. We are not putting together a deal. It is up to Haitians to decide if there's to be a change in government,'' the diplomat said on condition of anonymity.

Avril, who came to power in a soldiers' revolt in September 1988, promised political reform and elections after taking power, but he exiled seven opposition activists and arrested scores of people in a crackdown in January. Avril scheduled general elections this year, but most parties refuse to participate, saying they don't trust him to give up power.

The latest round of mass protests in Haiti began Monday after troops killed an 11-year-old girl studying on her porch.

On Friday, witnesses and radio reports said soldiers opened fire on about 250 students protesting at the state university, wounding three. Three people were killed Thursday, including a soldier beat to death by an angry mob.

Demonstrations also broke out Friday in the outskirts of Port-au-Prince and continued until sundown with protesters putting up flaming tire barricades and soldiers shooting in the air to break up crowds.

In the heart of Port-au-Prince, groups of heavily armed soldiers guarded the National Palace and two manned anti-aircraft guns were positioned on the palace's front lawn.

The capital of 1 million, usually teeming with people on weekend nights, was deserted Friday night and early today.

The remnants of smoldering tire barricades remained in many intersections and there was sporadic gunfire heard around the palace.