Moslem demonstrators protesting the Soviet presence in Afghanistan burned the Soviet flag and voiced support for the Afghan resistance movement during a peaceful march through downtown Washington yesterday.

About 50 demonstrators marched from the Islamic Center at 2551 Massachusetts Ave. NW through downtown, stopping briefly at 16th and K streets, half a block from the Soviet Embassy.

There a Soviet flag was unfurled and set aflame as the demonstrators, some clutching posters that read "Death to Communism" and "Justice and Hope for Mankind Is Islam," chanted anti-Soviet slogans.

A line of police officers stood across 16th Street, preventing them from getting closer to the embassy. Federal law prevents demonstrations within 500 feet of foreign embassies here.

The march ended with a brief rally on the steps of the Islamic Center, where speakers used bullhorns to call for the defeat of the 105,000 Soviet troops in Afghanistan.

"We want the Russians out of Afghanistan and we'd like the establishment of an Islamic government," said Mohammed Kayum, 34, who identified himself as a spokesman for the protesters.

Kayum, wearing a green arm patch bearing the name of the group sponsoring the march, the "Islamic Association of Afghan Students," said that the protest was timed to coincide with the anniversary of the Soviet invasion of Afghanistan in late December 1979.

Kayum, who said he is a native of Afghanistan, said the protestors supported the guerrilla organization headed by Burhannudin Rabbani, the leader of one of the two extreme fundamentalist groups in the Moslem country.

Afghani insurgents have been plagued with fierce factional disputes among several rival guerrilla organizations fighting the Soviets.

Kayum said the demonstrators wanted to "raise our voice" and let the guerrillas fighting in Afghanistan know that "we are with them."