About 150 Iranian students returned to the streets of Washington yesterday, protesting the continued rule of Shah Mohammed Reza Pahlavi and his choice of a new prime minister, Shahpour Bakhtiar.

Dragging an effigy of the shah, "smashed by the Iranian people," one demonstrator explained, they marched from in front of the White House to the Islamic Center at 2551 Massachusetts Ave. NW, chanting, "Down, down, down with the shah!" and "Death to the shah!"

Unlike Iranian demonstrators in years past, yesterday's protesters went without the paper masks that had been their trademark. One demonstrator, Mohammed Roshanaei, explained that the masks were discarded "as a symbol to show solidarity" with demonstrators in Iran who have turned out without masks to protest the shah's reign.

Now, Roshanaei said, savoring the irony of the turn in fate, agents of SAVAK -- the Iranian secret police -- appear in public wearing masks to protect themselves from future retribution from the shah's opponents.

Judging by demonstrations of the past 10 years in Washington -- and even some of more recent vintage by Iranian opponents of the shah -- yesterday's march was a relatively tame affair. When the students reached the Islamic Center, their continued progress to the Iranian Embassy, at 3005 Massachusetts Ave. NW, was blocked by about 40 D.C. police, helmeted and carrying riot sticks. No arrests were repoted.

After about half an hour of chanting anti-shah slogans the students turned and walked slowly back to DuPont Circle. Throughout the entire period, traffic along Massachusetts Avenue was backed up for several blocks as moterists slowed to watch the demonstration.

At one point, a demonstrator tried to hand a leaflet to two weell-dressed women in a passing Mercedes. The driver rebuffed the offer, shouting that she did not want the "Communist" literature. "We are not Communists," the young Iranian student protested. "We believe in God."

According to Roshanaei, who is studying computer science at the University of the District of Columbia and in national secretary of th Iranian Students Association, "any government acceptable to the shah is not accepted by the people. It's illegal." Their slogan, Roshanaei said, is "No U.S., No U.S.S.R. Iran is for the Iranian people. Freedom and independence in Iran."

The shah, his family "and everybody who is supporting the shah" should be arrested and tried by a "people's court" in Iran, Roshanaei said. If the shah is deposed, "we are asking the American people not to let him come here. He is a criminal."

As the students returned in an orderly line to Dupont Circle, they were adminished by a leader speaking to them over a bullhorn "to show Americans who we are."

Roshanaei expressed concern that recent reports of Iranian student demonstrators, protesting the presence of the shah's 90-year-old mother and sister in California, had portrayed the students as terrorists. "We are not terriorists," he said.