Waving banners, placards, flags and raised fists, thousands of demonstrators marched through the heart of downtown Washington to the Capitol yesterday to protest the policies of the Reagan administration.

District of Columbia police officials and the U.S. Park Police estimated that about 2,100 people participated in the demonstration, but the All People's Congress, the protest's organizers, insisted that the crowd was larger by several thousand people.

"We know for a fact that about 80 chartered buses from out of town brought in more than the police estimated," said Susan Lesser, a spokesman for the APC, a coalition of labor, student and social groups. "We estimated about 12,000 people."

Soaked in brilliant sunshine and soothingly warm temperatures, the protesters carried 6-foot-tall flags with slogans such as "Roll Back Reaganism" and "Jobs not war; U.S. out of El Salvador," as they marched.

Participants in the march, who came from as far away as Detroit, Chicago, Boston and New York and ranged in age from about 15 to 65, included representatives of groups fighting for the rights of American Indians, blacks, poor people, the handicapped, homosexuals, and Haitian immigrants.

The protest, one of a number held across the country as part of the National Days of Resistance, began at about 12:30 p.m. with a rally in Franklin Park at 14th and K streets NW. About a dozen speakers warmed up the group, including Sahu Barron of Washington, a national board member of the APC.

"We demand money for jobs," she said. "We demand money for food . . . we demand money for us, the people, the people who built the great wealth of this country."

Another speaker, Josephine Butler, chairman of the D.C. Statehood Party, took the opportunity to urge support for the statehood cause. "Join us in our struggle to free ourselves and to become equal citizens in this country," she said.

At about 2 p.m., the marchers, most of whom were dressed for the warm weather in blue jeans, shorts, sandals and tennis shoes, began a two-hour trek down 13th, along F, down Seventh, to E and First streets NW to the west side of the Capitol, where they heard more speeches.

Larry Holmes, a cofounder of APC, said in an interview that the diverse groups of marchers were united by opposition to the Reagan administration.

"We're going to keep the pressure on," he said. "We want a reversal of all the inhumane, antipoor, antilabor, promilitaristic policies and programs that the Reagan administration has pushed for in the last 15 months. We think that is possible."