Protests against South Africa's apartheid spread to seven cities yesterday as the AFL-CIO held the largest demonstration yet at the South African Embassy here, and picketed consulates from New York to Los Angeles to protest that country's jailing without charges of 21 black trade unionists.

In the largest turnout of protesters since the "Free South Africa" protests began Nov. 21, more than 500 demonstrators picketed the embassy here, and three national labor officials were arrested, including AFL-CIO secretary-treasurer Thomas R. Donahue. The second-ranking official of the labor federation, Donahue called for a ban on South African imports and a possible full-scale boycott if the white minority regime there does not change its policies.

The 13 million-member AFL-CIO has joined the two-week-old protest movement because of its longstanding support for the development of free trade unions in other countries, especially South Africa, where U.S. labor officials contend that low wages and poor working conditions amount to slave labor.

The arrests of Donahue, 56, along with Charles Perlik, 61, president of The Newspaper Guild, and Leon Lynch, 48, vice president of the United Steelworkers of America, bring to 22 the number of protesters arrested in what has become a daily ritual at the embassy on Massachusetts Avenue NW. Those arrested include seven members of Congress, along with labor and civil rights leaders.

Yesterday's actions were part of a stepped-up pressure by protest organizers, who have pledged to continue targeting the four consulates in New York, Chicago, Houston and Los Angeles, and eight "honorary" consuls in Boston, Mobile, Salt Lake City, Portland, Phoenix, New Orleans, Seattle and Cleveland.

In New York City, six people were arrested after they blocked the Park Avenue entrance to the South African consulate. Former New York secretary of state Basil Paterson was arrested along with two state legislators and several civil rights leaders after a demonstration by a labor coalition.

In Los Angeles, a contingent of more than 200 people representing labor and civil rights groups marched outside the consulate in Beverly Hills. "We're here to demand the immediate release of the union leaders," said Los Angeles City Council member Robert Farrell. There were similar but smaller peaceful protests yesterday in Chicago, Houston, Seattle and Boston.

In a related development, the South African Embassy confirmed yesterday that it plans to approve the Rev. Jesse Jackson's long-pending visa application to visit that country. But the announcement appeared to heighten concerns by some black leaders that such a trip by the unsuccessful candidate for the Democratic presidential nomination could be used by South Africa to divert attention from the growing anti-apartheid protests here.

Though two key leaders of the demonstrations, D.C. Del. Walter Fauntroy and Randall Robinson of the TransAfrica lobbying group sought to play down these concerns -- suggesting, instead, that the trip might help to build the protest, which they hope to expand to Europe -- the seemingly united front had crumbled by the end of the day.

Sources close to Fauntroy, whose relationship with Jackson has been chilly since Jackson failed to support him for an influential post in the Mondale campaign, said last night that members of the Congressional Black Caucus will meet with Jackson today to urge him to cancel what they see as a grandstanding trip.

"It's time to go beyond rhetoric," said one Jackson critic.

Jackson responded to the criticism coolly last night, saying he had been trying to get his visa for almost a year and pointed to planned South Africa visits by Sen. Edward Kennedy (D-Mass.) and the daughter of Atlanta Mayor Andrew Young.

The anti-apartheid campaign "is going to be a long struggle, I'm convinced, and we must try different levels of protest," Jackson said, adding that he still hopes to meet with President Reagan and Pope John Paul II on the issue.

But Jackson, who said he will be meeting with the Black Congressional Caucus today "on a broad range of subjects" for 1985, stopped short of saying he will carry out his announced plans for a January visit to South Africa. "If that is [the caucus'] recommendation -- not to go," that will not be a permanent recommendation not to go, he said. "We're not bound by a date, we're bound by a destination."

In addition to highlighting human rights issues, the ongoing protest "is important to American workers because they are losing jobs to the migration of jobs to Third World countries like South Africa," said Nana Mahomo, an exiled black South African who works for the AFL-CIO's African-American Labor Center.

"Multinational corporations are establishing factories [in South Africa] because of the lower wages, and if you promote unionism in Africa, you make it more difficult for companies to find offshore havens," Mahomo said.

AFL-CIO President Lane W. Kirkland was en route to Brussels yesterday for a conference of the International Confederation of Free Trade Unions, where he planned to push for international labor pressure against South Africa, a spokesman said.

Trade unions have been a focus in a recent wave of violence in South Africa that was among the worst in the nation's turbulent history. Union leaders played a key role in the Nov. 5-6 work stoppage by hundreds of thousands of South African black workers who stayed home in the biggest political strike by blacks in the nation's history.

Union membership by blacks was illegal and most union activities were banned until 1980, when South Africa instituted labor reforms. But critics charge that the crackdowns on union activists have negated whatever concessions the Pretoria government made to black labor.

The embassy, in a statement, said the detention since Nov. 5 of the union officials "were in no way connected with or related to legitimate trade union activities . . . The persons were detained in terms of various sections of the Internal Security Act which relates to activities causing a breakdown of law and order, including communication and transportation, or causing loss of life or damage to property.

Yesterday's arrests differed from what has become a well-scripted process because the South Africans closed the embassy. Instead of escorting the designated arrestees to the embassy entrance, D.C. police allowed the three labor leaders to cross the police line about a block from the embassy and then arrested them for crossing.

A police spokeswoman said the three chose to spend the night in jail at the 2nd District station.

Charging the three labor leaders with crossing police lines, a misdemeanor punishable by a $300 fine or 10 days in jail, puts their cases in the hands of the D.C. corporation counsel's office rather than the U.S. attorney's office, which has chosen not to prosecute demonstrators charged with coming within 500 feet of an embassy.

Since the D.C. corporation counsel's office has yet to dismiss charges against two other demonstrators, yesterday's action could open the way for the "show trials" that the protesters have been seeking to further publicize apartheid.