In a show of support for their comrades in Tehran, Iranian students led by two bearded mullahs briefly occupied the U.S. Embassy garden here today before being driven off by Syrian peacekeeping troops firing submachine-gun volleys into the air and furiously swinging rifle butts.

During the incident, Ambassador John Guther Dean and his staff were reported outside the embassy, which is normally closed for official business on Sunday.

The occupation lasted 2 1/2 hours and resulted in bloody heads for three of the demonstrators -- including both turbaned clergymen -- but no reports of any serious injury. The tough Syrian clearing operation was seen as a demonstration of resolve to let nothing happen here like the U.S. Embassy takeover in Tehran, where about 60 hostages are being held.

The approximately 40 Iranians, accompanied by a handful of Lebanese supporters, clambered over an iron fence surrounding the embassy's seaside garden, burned a U.S. flag and brandished posters attacking the United States and demanding that the former shah be returned to Iran. Lebanese security police at the embassy called in reinforcerments but did nothing to remove the slogan-chanting youths.

For more than two hours the demonstrators stood or sat on the embassy lawn shouting "Allah is great," "Khomeini is great" and demanding that the United States turn over the shah for judgment by the Islamic government of Ayatollah Ruhollah Khomeini in Iran.

Several hundred Lebanese out for a Sunday afternoon stroll stopped to stare and watch youths who had climbed on a roof over the embassy driveway to show off posters with photos of Khomeini and slogans such as U.S. return the shah."

U.S. Marine guards were seen on the roof of the embassy with walkie-talkies and light weapons. But they made no attempt to intervene against the demonstrators on the lawn. Lebanese security police stood between the shouting youths and the embassy entrance, blocking any attempt to move inside the fortifted building but doing nothing to expel them from the embassy grounds.

"We are here to show our support for the students who have taken over the embassy in Tehran," said one Iranian, who said he was a student at the Lebanese Arab University in Beirut. "We will stay here one day, two days, as long as it takes."

A Lebanese security officer said, however, that the mullah leading the demonstrators had agreed to say evening Islamic prayers and move the youths out by 7 p.m.

But soon after darkness fell, a contigent of Syrian peacekeeping forces pulled up. One group of soldiers moved the crowd back while another rushed into the embassy grounds firing bursts into the air and cracking heads with rifle butts. Within a few minuets the garden was empty, and the Syrian soldiers were chasing youths down side streets.

The Syrians, members of an Arab League-mandated peacekeeping force composed of more than 20,000 Syrian soldiers, set up patrols around the neighborhood and surrounded the embassy during the evening. There were no reports of armed opposition to their operation and no weapons were seen in the hands of the Iranian youths.

There was no sign of any Palestinian involvement in the Syrian operation. It was reliably reported last week, however, the Palestinian officials had prevented a band of Iranian youths from attempting to occupy the Beirut embassy shortly after the Tehran occupation began.