A weekend of parades and demonstrations begins here today with a parade of 10,000 school children and a march by Maoists advocating the "armed overthrow" of the U.S. government.

On Sunday, 20,000 demonstrators, mostly from the East Coast, are expected at an antinuclear "March on Washington" featuring a march down Pennysylvania Avenue from the Ellipse and a rally at the Capitol. Organizers of the rally said scheduled speakers include California Gov. Edmund G. (Jerry) Brown, Jane Fonda, Ralph Nader, Barry Commoner and others.

The weatherman says it will be partly cloudy and cool through Sunday. The chance of rain today is 20 percent.

The children are safety patrol kids from throughout the country and their parade begins at 10 a.m. today. The children will parade from Seventh to 17th streets NW along Constitution Avenue.

Then at 1 p.m. today the United Mayday Committee, including a pro-Maoist group that battled police to protest the state visit in January of Deng Xiaoping (Teng Hsiao-ping), vice premier of the Peoples Republic of China, will march to show that, "The flames of revolution are spreading."

The pro-Maoist Revolutionary Communist Party will be joined by four other groups, including Vietnam Veterans Against the War.

A United Mayday Committee spokesman said about 800 are expected on the march that begins on the 600 block of T Street NW and goes in a large circle for three hours before returning to the starting point.

At first the police department denied the committee a permit for the march, but then relented under instructions from the city Corporation Counsel office, a police department spokesman said.

Sunday's antinuclear march and rally was rapidly organized in the wake of the Three Mile Island incident.

"For a long time nuclear protest has been local," said one May 6 Coalition organizer, Tim Massad. "This march represents a strategic advance-mass action focused on Washington to build a broad-based coalition . . ."

Massad said the demonstration will establish that "opposition to nuclear power will be a major force in the politics of the 1980s.

Coalition head Donald Ross said he expects 600 bus-loads of protestors-a total of between 16,000 and 20,000 persons-to arrive in Washington this weekend.

The protestors plan to gather on the Ellipse between 10 a.m. and noon Sunday. There will be musical entertainment and some speakers there, then the march past the White House and down Pennsylvania Avenue to the west side of the Capitol.

There, between 1:30 and 5 p.m., the speakers will include Dick Gregory, Ralph Nader, Jane Fonda, Tom Hayden, Barry Commoner, Kurt Vonnegut, Bella Abzug and others. CAPTION: Picture 1, Volunteers at the May 6 Coalition headquarters discuss Sunday's rally. Up to 20,000 protesters are expected to arrive here this weekend. By Douglas Chevalier-The Washington Post; Picture 2, Michael Reinemer, left, Jeff Nix work on finding housinng for demonstrators. By Douglas Chevalier-The Washington Post