Thousands of demonstrators, carrying banners and chanting slogans, marched past the White House and rallied at the Capitol yesterday to call for change in Reagan administration policies on Central America, South Africa and a range of other issues.

The protesters came from throughout the Eastern Seaboard for the event, the centerpiece in a weekend of protests planned here by a coalition calling itself April Actions for Peace, Jobs and Justice, and one of several anti-administration demonstrations held across the country yesterday.

Filling Pennsylvania Avenue NW from curb to curb, and tying up downtown traffic, the demonstrators, including '60s veterans, college students and senior citizens, took more than an hour to wend their way to the Capitol for the final phase of yesterday's event.

U.S. Park Police and D.C. Police estimated at 26,000 the size of the peaceful and orderly crowd that gathered here under a broiling sun in temperatures well up in the 80s. The Rev. Jesse L. Jackson, a principal speaker at the concluding rally at the West Side of the Capitol put the figure at 100,000.

No arrests were reported. Personnel at aid stations said at least 40 persons were treated, mostly for heat-related problems.

Jackson touched on the range of concerns that brought the protesters here, including the administration's support for Nicaraguan rebels, its trade ties with South Africa and policy of "constructive engagement" there, and President Reagan's plans to visit a World War II German military cemetery.

"We must end the contradictions of funding the 'Contras,' trading with South Africa, honoring the Nazis and reuniting with the legacy of fascism," Jackson told the demonstrators, who swarmed west from the Capitol steps to the shade of the trees along the Mall.

"We will not be diverted by tributes at Bitburg," Jackson said, referring to the location of the cemetery Reagan plans to visit. "We are going to fight to free Johannesburg.

"It's time for a change.

"We want the real America to stand up, the America that all of us love and can be proud of."

A White House spokesman said the president was at Camp David during the demonstration, the largest held here against his policies in his second term. The spokesman said Reagan had no comment.

The demonstrators listened to Jackson and other speakers -- including representatives of a guerrilla group from South Africa and Nicaraguan university students -- express their views, and they voiced their own opinions through buttons, chants and banners.

Some of the signs and banners also identified the hometowns of the bearers.

"Peace to you from Kalamazoo," said the banner of one contingent. "Iowa City Wants Peace, Peace, Peace," read another. "Vermont Still Has a Dream," read a third. Other protesters carried a banner reading "Indianapolis Says No to White Rule in South Africa."

"Stop the bombing, stop the war; U.S. out of El Salvador!" protesters chanted as they marched past the White House.

A number of protesters also carried placards, apparently prompted by the planned cemetery visit, which linked the president to Fascism. "Hitler brought prosperity too," said one sign.

Another placard said "Bonzo goes to Bitburg."

Yvonne White and Vivian Delaney, both from New York City, said they got up at 4 a.m. yesterday to board a bus chartered by the Presbyterian Peace Ministry.

White said they came "especially for our seniors," referring to the elderly who she said have often been left hungry at the end of the month and forced to rely solely on inadequate social security payments.

"We see what the budget cuts have brought," said White, who said she traveled here instead of going with her daughter to pick out a veil for the daughter's wedding dress. "At some point you have to dig in your heels."

Yesterday's demonstration, which followed a rally Friday night near Thomas Circle NW, began with an assembly at the Ellipse, where six cultural arts programs drew demonstrators, as well as some curious tourists, in what seemed at times like a circus of causes.

Skits, speeches, poetry and songs, some given voice by protest veteran Pete Seeger, added to the festive atmosphere. A hawker shouted "get your ice cold buttons!" Obie Simonis, a sculptor from Cambridge, Mass., explained why a 50-pound, 14-foot tall fiberglass copy of a Pershing II missile was chained to his back.

In addition to making a statement about the nuclear threat, he said, the object was "to symbolize the economic burden of the arms race."

Jackson was in the vanguard of the march when the protesters, some bearing effigies of the president, some coffins to symbolize deaths in Central America, left for the Capitol. In front of the White House he knelt briefly in prayer.

"Use us in your own way to make this a world of peace, a world of justice," he said.

A few persons heckled the demonstrators as they passed. "Go back to Russia," a young man shouted.

For the most part, there were few hecklers, supporters or other spectators along the downtown streets. One onlooker, Bernie Radack of Silver Spring, said he was pleased to see the exercise of constitutional rights.

"I think it's nice that people can still come down and march for whatever cause they have," he said.

Yesterday's rally in San Francisco drew 50,000 persons, according to police there. In Los Angeles, about 4,000 persons rallied, and a protest was also held in Seattle.

Organizers said the April Actions demonstrations will culminate tomorrow at the White House with an effort to block entrances.

While the anti-Reagan group was marching yesterday, about 50 persons, led by the Rev. Carl McIntire of New Jersey, demonstrated in support of South Africa near the South African Embassy, where hundreds of persons have been arrested protesting apartheid.