As the people of embattled El Salvador prepare to go to the polls Sunday, thousands of critics of U.S. policy in that country, including folk singer Joan Baez and Argentinian Nobel laureate Adolfo Perez Esquivel, are expected in Washington for three separate protest meetings over the weekend.

The first demonstration, organized by the March 27 Coalition, a group of l6 committees opposed to U.S. policy in El Salvador, will begin today at ll a.m. in Meridian Hill Park at l6th and Euclid streets NW. From there,the protesters will march through the largely Hispanic neighborhood of Adams-Morgan and down Connecticut Avenue to Lafayette Park where a rally will begin at 3 p.m.

A spokesman for the protest organizers said he expects more than l0,000 people to attend the demonstration.

At the same time, a socialist group called the Anti-Imperialist Coalition, which advocates a military victory for the Marxist insurgents, will hold its own rally in Farragut Park at 2 p.m.

Baez and Esquivel, who won the international award in l980 for his human rights work in Argentina, are among those scheduled to participate in an ecumenical prayer service Sunday at 5 p.m. in the New York Avenue Presbyterian Church. The service commemorates the second anniversary of the assassination of Salvadoran Bishop Oscar Romero.

Yesterday a group of about 500 youths associated with the Collegiate Association for Research of Principles, the youth arm of the Rev.Sun Myung Moon's Unification Church, marched from Capitol Hill to the White House to show their support for what a spokesman called "the democratic element in El Salvador and the administration's centrist stand."