Three civil rights leaders arrested earlier this week during a sit-in at the South African Embassy here said yesterday they are forming a new "Free South Africa Movement" aimed at pressuring the Reagan administration to change its policy toward the white-minority-controlled country.

At a news conference on Capitol Hill, the three said labor, church and political groups will be involved in "direct action," including daily sit-ins and demonstrations, at the embassy and at South African consulates throughout the country.

"We want to focus attention on the policies of South Africa and the Reagan administration's embrace of them and put this in the middle of the American policy agenda," said Randall Robinson, executive director of the lobbying group TransAfrica and coordinator of the Free South Africa Movement. "We are challenging Ronald Reagan's policy of constructive engagement, which only gives comfort to an oppressive regime as its policies worsen."

The term "constructive engagement," coined early in the Reagan administration, was based on the idea that former president Jimmy Carter's policy of treating South Africa as a pariah had not helped anyone inside the white-ruled nation.

The Reagan administration, while saying that it regards South Africa's apartheid policy as abhorrent, believes that the way to influence South Africa's policy is to take a more piecemeal approach, seek improved relations and urge reforms over a longer period of time.

The announcement yesterday followed the arrests Wednesday of Robinson, D.C. Del. Walter E. Fauntroy and Mary Frances Berry, a member of the U.S. Civil Rights Commission, who had staged a sit-in at the South African Embassy to protest the recent arrest of thousands of blacks involved in a labor strike in South Africa.

Organizers of the newly formed movement said their immediate goal is to secure the release of 13 South African labor union leaders who have not been heard from since they were arrested during the two-day strike Nov. 5 and 6.

Others whose release is being sought include black South African leaders Nelson Mandela and Walter Sisulu, who have been imprisoned since the early 1960s.

Although supporters of the group are making South Africa's racist policies their rallying point, organizers say this is not a "black movement," and issued an appeal to white trade unionists whose interest would also be hurt by "slave labor pools" of South Africa.

"When American corporations can relocate to places where there is cheap labor supply, jobs are lost in America," Berry said. Calling the Reagan administration "destructively disengaged from morality," she said, "We just want our government to be on the side of freedom, not oppression."

The first in a series of demonstrations is scheduled for 3 p.m. Monday in front of the South African Embassy at 3051 Massachusetts Ave. NW and others are expected to be held everyday at that time. Howard University students have planned a campus demonstration on Dec. 1.

The "direct action" approach to protest marks a dramatic change for the leaders of the Free South Africa Movement, most of whom had been deeply involved in the civil rights protest action of the 1960s but later settled into more traditional roles of lobbying and shaping legislation.

Contending that those approaches have not worked in affecting U.S. relations with South Africa, the organizers said a "new level" was necessary.

"The action already taken and those planned have been arrived at reluctantly," Robinson said. "But the circumstances in South Africa and the support of this country have demonstrated that a direct action is necessary, meaning any measure that will focus American attention to this problem and cultivate an American understanding and sympathy for those who suffer much in South Africa. Black leadership . . . has reached a point where it is willing to return to those measures that produced results in the past," he said.

Robinson, Fauntroy and Berry were released from the D.C. Jail on Thursday after pleading innocent to charges of unlawful entry in the sit-in at the embassy. D.C. Superior Court Judge Bruce S. Mencher set Dec. 20 for a pretrial hearing in the case. The three were arrested by U.S. Secret Service officers after they refused to leave a meeting with South African Ambassader Bernardus Fourie about the fate of the detained labor leaders.