Three cars slowed in front of the Embassy of the Islamic Republic of Iran on Massachusetts Avenue yesterday as the drivers waved fists and honked their horns at the building in a futile protest against the holding of 50 American hostages in Tehran.

Sgt. Clarence A. Robinson in charge of a two-man force of U.S. Secret Service uniformed police assigned to guard the embassy was unimpressed.

"They do that all the time," said Robinson as he kicked at a pebble with a look of boredom on his face. "That's the most exciting thing that has happened all day."

Robinson and his partner, officer Thomas S. Thayer of Northeast Washington, were trying to convince themselves that the embassy shift was not such a bad way to spend Christmas Day.

"It's not so bad . . . could be raining. It could be cold and snowing. I mean it's real quiet out here," Robinson said. "It's slower than most Sundays today. I'll still get to spend the evening with my family at home."

An Iranian Embassy official climbed into a brown Mercedes and drove past the two officers without a gesture or acknowledgement.

"This is just our day for this duty," Robinson said. "They don't say anything to us and we don't say anything to them either."

Robinson, a Manassas, Va., resident, said duty in front of the Iranian embassy has so far been without incident.

"We're assigned to cover the Israeli and Russian embassies, too," said Robinson, who has worked with the foreign missions branch of the Secret Service for the past year. "It hasn't been any different. It's all about the same, standing in front of a building."

Thayer rubbed his eyes and waited for the end of the team's 7 a.m. to 3:30 p.m. shift.