A key leader of apartheid protests at the South African Embassy and elsewhere said yesterday that limited sanctions announced by President Reagan against the racially segregated nation are "inadequate and toothless" and should not be allowed to derail tougher economic sanctions pending in Congress.

Randall Robinson, coordinator of demonstrations that began focusing national attention on apartheid 10 months ago, said supporters on the Hill are still hopeful that the Senate will end its filibuster against a more restrictive sanctions bill.

"This is one of the most historic pieces of legislation to come before the Senate in recent years," said Robinson, executive director of the TransAfrica foreign policy lobby here. "A vote against cloture cutting off debate on the bill is a vote against social justice."

Since Robinson and two others were arrested during a sit-in at the embassy Nov. 21, apartheid protests have sprung up in 21 cities and on college campuses around the country. About 4,000 demonstrators, including more than 3,000 at the embassy, have been arrested.

"This is going to go on," said Robinson, who met with other antiapartheid leaders yesterday to help plan more demonstrations -- at the embassy and elsewhere -- toward the end of the month.

He called Reagan's executive directive a "further gutted" version of the stronger sanctions Congress was preparing to approve, sanctions he said were flawed by compromises with the Republican-controlled Senate but "still a step in the right direction."

He said the Pretoria government's condemnation of Reagan's measures is "part of the theater . . . the de facto alliance between them and this administration is still in effect."

Antiapartheid leaders, comparing Reagan's measures with those pending in the Senate, said Reagan's directive puts off banning the importation of krugerrands, effectively scuttling the proposal, and imposes no deadline for making further racial reforms to avoid future sanctions. They were especially critical of the "exceptions" the president has made for loans and exports that would improve the welfare of those disadvantaged by apartheid.

In addition, Robinson noted Reagan's "striking omission" of any reference to the Rev. Allan Boesak; Trevor Tutu, son of Anglican Bishop Desmond Tutu, or others who have been arrested and are now detained by the South African government because of their antiapartheid activities.

"I told Bishop Tutu on Saturday that as long as his people suffer, we will be out here," Robinson told a group of about 100 Presbyterians from North Carolina who were demonstrating against apartheid yesterday at the embassy.