Nobel Peace laureate Desmond M. Tutu announced tonight his refusal to join a delegation of church leaders scheduled to see President Pieter W. Botha Monday because of his disgust at the president's hard-line speech Thursday, which dashed hopes of any change soon in South Africa's apartheid system of white-minority rule.

Tutu, who was spurned last month when he sought an urgent meeting with Botha to try to defuse the unrest in his country's black townships, said in a telephone interview that the president's defiant speech was "the final nail" in his attempts to conciliate.

"It is clear from Thursday's speech that President Botha does not want to meet with any black leaders except those of his own choice," Tutu, who is the black Anglican bishop of Johannesburg, said after conveying his decision to the head of his church, Archbishop Phillip Russell. The archbishop will lead the nine-person delegation from South Africa's major Christian faiths.

Tutu today also stepped up his criticism of the Reagan administration's policy toward South Africa known as constructive engagement. Speaking earlier on CBS's "Face the Nation," he said the policy is "as evil, as immoral, as un-Christian as the policy they are seeking to buttress -- apartheid." Describing the volatile situation in his country, Tutu said, "I've never felt more sense of foreboding . . . . I think we're on the brink of disaster unless America intervenes."

While the political and diplomatic focus of the last few days has been on Botha's remarks and the chorus of domestic and international criticism they engendered, violence and arrests have continued.

One more person was killed and at least 25 were injured in black townships today amid reports of conflict across a wide front in all four of South Africa's provinces. At least 38 persons were reported arrested.

More than 150 persons were arrested yesterday in one of the largest detention sweeps since the government's state of emergency was declared July 21 in 36 towns and cities in an attempt to crush the racial unrest that has disrupted South Africa for nearly a year and caused more than 600 deaths.

Fears of fresh violence were raised today when the mother of a black radical scheduled to be executed Wednesday appealed for international pressure to spare his life.

The activist, Benjamin Moloisi, 31, is due to be hanged for the slaying of a black security police officer more than two years ago, but his mother and lawyer both protested his innocence today and warned that if the execution went ahead it could inflame feelings among blacks.

"There is so much tension now following the president's negative speech that this could easily spark further trouble," lawyer Priscilla Jana said.

Despite his critical remarks today, Tutu said he is still ready to play a mediating role if Botha shows any sign of changing his attitude.

Tutu sought an urgent meeting with Botha soon after the government declared the state of emergency. After initial encouraging signs from the president's office, Botha responded to Tutu's request by saying that his schedule was too tight to include an early meeting with the black bishop and implying that Tutu could join Russell's delegation.

Botha compounded the snub by including a caveat making it difficult for Tutu to join the delegation, saying he was willing to "meet with anyone who renounces violence and civil disobedience."

Although Tutu is strongly opposed to violence and has risked his life several times to stop angry mobs from killing blacks they regard as "collaborators" during political rallies, he favors strategies of civil disobedience and foreign pressure on South Africa.

Tutu's immediate reaction to Botha's conditional offer was to refuse, but he came under strong pressure from church and other moderate sources to join the delegation, and by last week it seemed likely that he would do so. Then on Thursday night, after expectations had been built up that the president was going to announce major changes, Botha delivered a tough, defiant speech warning the world not to try to pressure South Africa and declaring that he was willing to talk only with "elected" black leaders.

"First of all, I had hoped for a one-on-one session at which there is some possibility of a meeting of hearts and minds," Tutu said tonight. "I was really not interested in a group meeting, when one is facing a gallery."

Noting that Botha subsequently had granted several short-notice meetings, Tutu said that he was offended by the fact that the president "obviously did not speak the truth" when he said he could not fit him into his schedule.

But he added: "I still want to meet him. The situation in South Africa is too serious for me to turn my back. I am still willing to talk to him if he will agree to a one-on-one meeting and if he does not attach conditions which he knows I can't accept."

In today's violence, a woman was killed in Bongweni black township, near the eastern Cape Province farming town of Bedford, when she was run down by a truck that was being pursued by a rioting mob, police said. Black residents of a township outside the port city of East London, scene of growing violence during the past four days, said 25 persons were injured when police opened fire with shotguns on a crowd at the home of an earlier victim.

Reporters said a tense situation developed in the western Cape Province town of Worcester today when police at a roadblock refused to allow the Rev. Allan Boesak, a mixed-race clergyman who plays a leading role in the black activist movement, to enter a township to visit the family of a youth killed in a clash with security forces Friday.

If Moloisi is hanged Wednesday, it will be the fifth execution of a member of the underground African National Congress, which is trying to overthrow white-minority rule by guerrilla struggle. The other four have been lionized by the black underground, and the executions have aroused strong emotions in the black townships, where the guerrillas are increasingly regarded as heroes. This has raised fears of trouble if Moloisi is executed in the midst of the present tension.

Moloisi's mother, Pauline Moloisi, 53, appealed today for international pressure to save her son, who she insisted was innocent of shooting a black security police officer, Phillip Selepe, with an AK47 rifle in Mamelodi township outside Pretoria in November 1982.

Pauline Moloisi noted that the ANC had issued statements admitting that it had sent a hit squad into South Africa to kill Selepe but that her son was not a member of the squad. Jana, Moloisi's lawyer, accused authorities of "some serious irregularities" in handling of the case.