When three Washington civil rights activists first staged a sit-in at the South African Embassy nearly 10 months ago, little did they know that they would spark a storm of defiance resulting in a bid to impose economic sanctions on Pretoria.

But that is exactly what has happened, and it now appears certain that some form of sanctions will be imposed. The Senate is expected to approve sanctions legislation, which would include bans on new U.S. bank loans to South Africa as well as on the export of nuclear and computer technology.

President Reagan, who probably will veto it, intends to take executive action today imposing weaker and more limited measures against South Africa to try to avoid a major defeat on the legislation. Such action, in my view, would be an affront to the American people, whose will is being reflected in the congressional action.

Whatever the outcome of this week's actions, however, the enormous debt owed to the Free South Africa Movement, which has become one of the most significant civil rights achievements in two decades, cannot be denied. Jean Sindab of the Washington Office on Africa said after the first Senate vote: "The high visibility of the issue of apartheid generated by the FSAM demonstrations and the constant pressure maintained by its grass-roots allies have forced the conservative, Republican-dominated Senate to pass this [sanctions] bill."

It was last Thanksgiving eve when Randall Robinson, head of the lobbying group TransAfrica, Del. Walter Fauntroy (D-D.C.), Mary Frances Berry of the U.S. Commission on Civil Rights and Eleanor Holmes Norton of Georgetown University Law School sat in at the Massachusetts Avenue embassy. Norton left the embassy before the arrests to notify the press and public. Robinson, Fauntroy and Berry were arrested.

"It was a substantial step," Robinson remembers. "I had never been arrested before but for between 10 and 20 years we have worked to get the American body politic and public to give South Africa the kind of priority it deserves."

Three days later, they announced formation of a "Free South Africa Movement," involving labor, church and political groups in direct action -- including weekday demonstrations at the embassy. "We are challenging Ronald Reagan's policy of constructive engagement, which only gives comfort to an oppressive regime as its policies worsen," said Robinson, who became the group's coordinator.

In the wake of growing concern about the oppression from South Africa's constitutionally enshrined racism, protests spread like brush fire across the country -- to South Africa's consulates in 13 cities and to college campuses. The protests crossed party, ideological and racial lines, drawing strength from churches, local governments and ordinary Americans.

Late last year, 35 House Republicans wrote a letter of protest to the South African ambassador. Twenty-two members of Congress, both blacks and whites, have been among some 3,000 people arrested in Washington, as have Coretta Scott King and Amy Carter, daughter of the former president. The demonstrations and the arrests spread to 26 other cities; about 4,000 people have been arrested nationwide.

Although 70 percent of those arrested have been white, black leaders have been in firm control of the movement, reviving some of the same tactics that proved so effective during the 1960s civil rights struggle and racking up one of the most substantial civil rights achievements in the last couple of decades.

Despite similarities between the Free South Africa Movement and the civil rights struggle, the movements differ in two key ways:

More women are involved in the leadership. Berry and Sylvia Hill are key members of the steering committee

And unlike the '60s, the movement's leadership is not church-based, although Fauntroy is a minister. Roger Wilkins, another Free South Africa leader, is an author and journalist. TransAfrica is a lobby on foreign policy issues. As events in South Africa continue to escalate in bloody and brutal conflict, things are certain to get worse before they get better. And while FSAM leaders are careful not to be too jubilant as they near a first victory, they can take pride that a movement of humble beginnings has had considerable impact on the nation -- and the world.