U.S. Marine guards fired tear gas to disperse a mob of youthful demonstrators today after an American Embassy car was set afire outside the embassy compound in downtown Tehranm officials said.

An embassy spokesman reported no injuries in the incident. But withnesses said four youths were shot in the leg by Iranian troops in separate demonstrations across the city in which several thousand people were estimated to have taken part.

Political observers said it was too early to tell whether the demonstrations marked the start of another round of upheavals in iran's year-long wave of religious and political opposition aimed at dethroning Shah Mohammad Reza Pahlavi.

They pointed out, however, that the destructive riots in Tehran last month that led to installation of a millitary government started after clashes between troops and student demonstrators. Severals university students were killed Nov. 4 when soldiers opened fire on a mob that pulled down a statue of the shan on the Tehran University campus. The incident helped set off rampages the next day that destroyed hundreds of banks, government buildings, movie theaters and businesses.

The protest today coincided with a government decision to close more than 60 secondary and technical schools only a day after they reopened. The schools had been closed more than a month because of the earlier disturbances. The reopening yesterday was met with student and teacher strikes and a series of demonstrations that escalated today.

The capital's universities remained closed because of government fears that their reopening would unleash more disturbances.

Tehran's martial law administrator, Gen. Gholam Ali Oveisi, said one army officer and several soldiers were injured when agitators among the demonstrating students threw a hand grenade during disturbances in which official and military vehicles were set on fire. No independent confirmation of his charge was available.

Other protesters swarmed through the main offices of the government owned Iran Air, damaging the airline's computer, Associated Press reported. A company spokesman, Akbar Nazemi, said damage was expected to run into the millions of dollars.

In the northeastern city of Mashad, tens of thousands of persons marched through the streets today to mourn the victims of an army shooting the day before. Diplomatic sources said between 8 and 12 persons were killed when troops opened fire on anti-government demonstrators gathered at the house of Mashad's top religious figure, Ayatollah Shirazi.

The incident at the U.S. Embassy occured around midday when about 200 youths, mostly of high school age, marched down the street in front of the walled compound. According to embassy sources, they began shouting anti-American slogans, chiefly "Yonkly Go Home," and threw rocks and burning objects over the wall.

Then they overturned and set fire to an embassy Chevrolet in front of a gate, officials said. At least one youth tried to climb over the wall, but he was forced back, they said.

When nearby Iranian Army troops stationed at a corner outside the embassy about 100 yards away from the mob did not immediately intervene, Marine guards fired tear gas from inside the embassy compound, sources said. Iranian soldiers later arrived and also shot tear gas canisters at the crowd, they said.

"If the troops at the corner had moved a little faster, the whole thing wouldn't have amounted to a hill of beans," one source said.

The antigovernment protesters have been increasingly turning their wrath on Westerners, especially Americans, whom they accuse of propping up the shah's government. The anti-Western sentiment has been behind a number of firebombings in recent weeks against the cars and homes of Americans living in Iran.

Saturday, the top American oil man in the country, Paul E. Grimm, was assassinated by terrorists in the southwestern city of Ahwaz. A team of embassy investigators flew down to the site to conduct an inquiry and try to set up procedures to tighten security for the remaining Americans there.