The 150 Vietnamese demonstrators in Lafayette Park had just begun to sing the old anthem of the Saigon regime, waving a small sea of its yellow and red flags above their heads, when a scarlet-masked throng of 350 Iranian Moslem students appeared marching toward them down Pennsylvania Avenue.

"Down, down, down with the shah," the Iranians shouted. During their brief encounte the two groups spent most of their tiem handing leaflets to each other.

It was Human Rights Day, the 30th anniversary of the Universal Declaration of Human Rights, and the streets and parks near the White House became a stage for groups from all over the world, including may refugees from oppression and persecution who were imploring the United States government to help their countrymen or, at least, to stop helping the people they feel are oppressors.

Before the day was out, a group of about 100 Iranian businessmen and doctors from all over the Midwest and the East Coast would join the students protesting before the White House. As those groups broke up and drifted away, South Koreans opposed to the dictatorship of Park Chung Hee and Filipinos against the regime of Ferdinand and Imelda Marcos would take their place in the fading winter light and near-freezing air.

Nearby, at 16th and K streets NW, within sight of the shuttered gray Russian Embassy, members of Amnesty International stood shivering from dawn to dusk with signs protesting the imprisonment of religious activists -- Protestant and Catholic as well as Jewish -- in the Soviet Union.

The District of Columbia and U.S. Park Police were ready for any trouble that might arise -- including a bus full of officers in riot gear on each side of the park -- but all the demonstrations went off without incident.

The police were left to joke among themselves and stroll through the crowds. "You can't tell the players without a scorecard," said one park policeman. "The Eskimos are coming later to complain about insufficient heat in their igloos," suggested another.

Most of their serious attention, however, was concentrated on the Iranian students, who went about their demonstrating with diligent, almost professional care. A gas-powered electric generator was brought in to power the huge amplifier and speakers blaring their condemnation of the shah.

Their protest, as one masked spokesman for the Organization of Iranian Moslem Students put it, was a "reflection of the demonstrations in Iran" that took place on a massive scale yesterday and are expected to continue today, surrounded by fears of major violence.

Many regared the moment when the students were joined by the doctors and professionals, who had been demonstrating separately at first, as a significant gesture.

"This showed probably for the first time the organized opposition of the professionals and physicians," said Dr. Mohammad Hashemi, an Iranian-American radiologist from St. Louis. "It shows that opposition to the shah is not just in a special group."

The vast majority of Iranians protesting yesterday were united moreover by their Islamic faith. At one point during the safternoon several knelt in prayer towards Mecca. Today more demonstrations are expected by Iranian student groups without religious affiliations.

The Vietnamese demonstrators were mostly people who fled their country after the collapse of the Saigon regime. Some were former government officials, others ordinary working people. Some had escaped from Vietnam on boats as recently as last summer.

"A lot of these people were anti-Thieu, and they wouldn't be here if they thought they had a choice," one marcher, who asked that he not be identified, said of former Vietnamese Premier Nguyen Van Thieu. "It's not that everyone wants a change in the present government, it's tht they couldn't stand it anymore."

Others advocated the overthrow of the Hanoi government. One, a former Saigon bureaucrat named Huynh Tan Le, said he had been part of the poorly armed and supplied National Restoration Movement before he bought his way out of the country on a fishing boat last June with 400 grams of gold he had saved during the Thieu regime.

All concurred, however, with the letters to President Carter and United Nations Secretary General Kurt Waldheim asking them to take positive action to improve the current human rights situation in Vietnam. The letters were read after the demonstrators marched to the Capitol. By then the ranks of the demonstrators had swelled to more than 200.

By comparison, the South Koreans and the Filipino demonstrators who joined them in front of the White House fence toward sunset seemed almost forlorn. They called for the freeing of political prisoners such as Korean poet Kim Chi Ha and the overthrow of the Park and Marcos regimes. Altogether they numbered only about 70 people.

Some of the Koreans, adopting a tactic that has served the Iranians well for yers, donned cloth masks to protect their identities and symbolize the oppression they oppose. At one point two vans full of Iranian students, leaving their own demonstration, stopped briefly to cheer them on.