Thousands of demonstrators opposed to the nuclear arms buildup, domestic budget cuts, apartheid and U.S. involvement in Central America will converge on the nation's capital over the next four days for protests at the White House and around the city and for lobbying in the halls of Congress.

The long weekend of what is being called "April Actions for Peace, Jobs and Justice" will begin this afternoon at the South African Embassy, where hundreds of church leaders and their congregations are expected to join the continuing protests against racial domination in the white minority-ruled nation.

On Saturday, protesters will stage a cultural arts "Festival of Resistance" on the Ellipse, followed by a march down Pennsylvania Avenue to a rally at the Capitol.

The demonstrations will culminate Monday at the White House, where prominent figures from the Vietnam protest era and others will be trying to block entrances to the executive mansion.

On Capitol Hill, a citizens lobby will spend the day talking to House and Senate members on behalf of specific legislation that would curb or halt current U.S. policies.

"President Reagan and his colleagues have mastered the 'big lie' technique in the conduct of domestic and foreign affairs," Damu Smith, national coordinator of the April Actions coalition, said yesterday at a news conference outlining the schedule. "In too many instances, Congress has bought it, and the media has sold it."

Smith said the demonstrations are an attempt to form a multi-issue coalition bound together by opposition to U.S. policies.

Pointing to recent apartheid protests, the church-sponsored sanctuary movement for Central American refugees and the growing militancy among farmers threatened with bank foreclosures, he said increasing numbers of Americans seem ready to stand up to the government.

But the task of redirecting American policies and priorities was called "monumental," and organizers of the weekend protests said yesterday that their efforts are just getting under way.

"This demonstration is simply the tip of an iceberg of conscience," said the Rev. Paul Mayer, executive director of the interreligious task force for the New York-based Mobilization for Survival.

Organizers said they expected "tens of thousands" of demonstrators for the weekend protests. In applying for demonstration permits, according to the U.S. Park Service, the sponsoring group have indicated that they are hoping for up to 50,000 participants on Saturday.

While noting that the planned events are reminiscent of the civil rights and antiwar protests of the 1960s, the Rev. William Davis, codirector of the Christic Institute, said there is one significant difference: "People are much more aware of the linkages [between domestic and foreign policies] and the lies."

Davis, a Jesuit priest, said it is getting harder for the Reagan administration to say it is fighting for democracy in Nicaragua while it bolsters the apartheid government of South Africa.

"This may be our last chance to be the country we say we are," Davis said.

Ron Covig, a Vietnam veteran and author whose war wounds require him to use a wheelchair, said the government didn't take care of its Vietnam veterans adequately after the war and would not take care of any new generation it might send into combat in Central America.

In describing Saturday's "Festival of Resistance," organizers said the event will begin at 10 a.m. and feature music, dance and other expressions of opposition to U.S. policies.

One feature will be an exhibit of "Spectacles," transformable sculptures depicting Uncle Sam as Pac-Man "gobbling up Central America", and a giant krugerrand gold coin that will open to display a mural of the struggle against racism.

Parallel activities are also planned for this weekend in San Francisco, Los Angeles, Seattle and Houston.