Secretary of State George P. Shultz yesterday strongly condemned South Africa's repressive tactics "that, in recent weeks, have caused many deaths," but he argued that the United States should continue trying to influence the white-minority government to "answer the demands of the black majority with negotiations."

"We can do our job only if we are engaged, physically present, in contact with all parties and able to bring our limited influence to bear," Shultz said in a speech to educators from predominantly black American colleges.

"The alternative -- disengagement -- will only reduce our ability to achieve our goals and make us less relevant to this important region," he said.

Shultz was replying to calls for the Reagan administration to end its ties with South Africa and seek, through economic and political sanctions, to end that nation's repressive system of racial segregation.

"We shouldn't just throw up our hands and say, 'We don't like what's going on here, so we're going to leave,' " he argued. "We should say, 'We don't like what's going on here, so we are going to be engaged and help to change things.' "

Shultz's comments were a restatement of his testimony before a House subcommittee March 21. At that time, his criticism of the Pretoria government was overshadowed by President Reagan, who told a news conference that night that rioting in South Africa might have been influenced by outside agitators seeking to overthrow the government.

The apparent contrast between their remarks led many black leaders and human-rights activists to charge that Reagan appeared to condone the South African actions and did not support the condemnation voiced by his secretary of state.

In an interview yesterday with The Washington Post, Reagan denied that he was trying to excuse South African actions or "voicing a bias." But he repeated his contention that "there is an element that wants to overthrow the government by violence . . . ."

In the interview, Reagan also agreed with Shultz's acknowledgement that "apartheid is the main problem that must be resolved," and he echoed Shultz's argument that "just walking away would leave us with no ability to influence them."

Shultz, in his speech yesterday, said, "President Reagan has made clear our position that South African apartheid is an affront to every principle and ideal of our country."

He also recalled that, last Dec. 10, the president publicly called on South Africa's leaders to end repression of the black community and "move toward a more just society."

"South Africa needs peace, not violence; dialogue, not confrontation and repression," Shultz said.