TEHRAN--Guerillas firing automatic weapons stormed the U.S. Embassy in downtown Tehran today, wounding two Marine guards and taking Ambassador William H. Sullivan and 101 other Americans hostage before forces of the new Iranian government rescued the Americans after a 3 1/2-hour gun battle.

Later reports said armed gangs were searching American homes in Tehran and, other Iranian cities. No further injuries were reported.

The Carter administration plans to resume evacuation of Americans from Iran this weekend and ultimately to airlift 500 U.S. citizens from the embattled country, sources said today.

The evacuation is intended to reduce the American community in Iran--officials, military contractor employees, businessmen, teachers and others--to no more than 2000.

U.S. officials said 1700 of the nearly 7000 American citizens currently in Iran are prepared to pull out as soon as the Tehran airport, currently shut by the new regime of the Anatollah Ruhollah Khomeini, reopens. The airport may reopen Saturday after observances of Islamic holidays.

A U.S. spokesman commended the Khomeini regime for acting quickly to free Ambassador Sullivan and the others and added that Assad Homayoun, the head of the Iranian embassy in Tehran, telephoned assurances from the Khomeini government that the army would protect the Americans.

Possible Attack

Had the Khomeini regime not freed Sullivan and the other hostages, the U.S. government might have had to take military action to protect the Americans, a U.S. official said.

American officials are closely watching the situation in Iran, Rex Granum, the deputy White House press secretary, said yesterday. A power failure has prevented all but fragmentary communications between Washington and the embassy.