Gunmen raked an American University Hospital bus with machine-gun fire in west Beirut today, killing two doctors, a hospital administrator and a student in the first major challenge to a three-week-old Syrian-backed security plan aimed at pacifying the predominantly Moslem sector of the capital.

A blue Peugeot sedan intercepted the red and white, 32-seat bus at Barbir Bridge only yards from an Army checkpoint between west and east Beirut. The bus was carrying Christian doctors, students and nurses who were returning home to the Christian sector from work in the west the city.

"We just ran into a hail of gunfire. All I can remember is being showered with bullets," said the dazed bus driver, his T-shirt and trousers stained with the victims' blood. A doctor, apparently in shock, sat frozen on one of the seats staring straight ahead. He had helped carry the victims to the Maqassed Hospital in west Beirut to which the bus had returned with the victims.

The windows of the bus were blood-streaked and a red pool covered the floor of the vehicle as it sat outside the hospital.

After the attack, the gunmen sped away and disappeared into the Ras Nabe district, a mostly deserted residential area, which is a stronghold for fundamentalist, Moslem and leftist groups.

Workers and doctors at the American University Hospital were stunned by the daytime assault, which drew denunciations from both Moslem and Christian leaders in Beirut.

There was no solid indication of who was behind today's violence, although there are many parties who stand to lose from Syria's new initiative and military presence here. On July 4, 300 Syrian special forces and scores of other Syrian intelligence agents were deployed around west Beirut to bolster a security plan for the chaotic Moslem sector.

An outcry by Moslem and Christian leaders against today's ambush signaled fears of new internecine fighting and a collapse of the security plan, which had raised cautious hopes here.

Education and Labor Minister Selim Hoss, a former prime minister, said, "Those committing such hideous acts are the enemies of humanity in this country. Those targeting doctors and nurses are mercenaries and criminals."

One of the doctors killed was Nelly Khairallah, who graduated only one week ago, hospital spokesmen said. The other was George Phares. Also killed were hospital administrator Wadih Abu Khalil and a student, Lilliane Abdullah Badawi. Two other doctors, Michel Rbeiz and Fadi Qadhifa, and a student, Elie Estphan, were wounded.

More than 70 doctors, mostly Christian, have left the American University of Beirut, some after being warned to leave west Beirut amid a wave of attacks against western interests that began last year. The hospital's director, David Jacobsen, an American, is among the foreigners missing and believed kidnaped in Lebanon.

Some Christian doctors who had been driven out earlier this summer were contemplating returning after the security plan appeared to be taking hold. Minor violations -- sticks of dynamite hurled into garbage piles, rocket-propelled grenades fired in the direction of a seafront hotel housing Syrian military observers, and one or two kidnapings -- had not succeeded in marring the new climate of hope.

The university bus that was attacked today shuttles between east and west Beirut three times a week in what is supposed to be the safest trip for commuting hospital and university staff and students.

"It was all a matter of seconds. If I had not put my foot down on the accelerator, all of us would be dead," the bus driver said.

The Lebanese Forces, the main Christian militia, said it held Syria directly responsible for the assault on the bus.

The attack, which Christian media described as a massacre, was reminiscent of an April 1975 assault against a bus carrying Palestinians at the beginning of Lebanon's civil war that sparked clashes between Christians and Moslems and drew the Palestinian guerrilla movement into Lebanon's religious conflicts.