The South African government continued rounding up black activist leaders today as key figures in South Africa's unfolding drama appeared to be setting the scene for a political challenge by moderate forces to the hard line being taken by President Pieter W. Botha against negotiations with black radicals.

Police arrested five more leaders of the activist United Democratic Front today, bringing to 32 the number from the organizaton detained without charges under the stringent security laws since Friday.

As the arrests and political maneuvering continued, a poll of South African blacks released in London showed that 77 percent of those questioned support international economic sanctions against Pretoria's apartheid policies.

In the poll, commissioned by the London Sunday Times and conducted by the South African affiliate of the Gallup Poll, 69 percent said they believed the country was headed for all-out civil conflict, even though there was widespread opposition to the idea of violence.

Government officials today also refused a clemency plea for an elderly and sickly black leader who was taken to prison in a wheelchair to begin serving a five-year sentence.

Justice Minister Jacobus Coetsee said he could not grant clemency to Oscar Mpetha, 75, who has endured two amputations and has failing eyesight, because the ailing leader, a former copresident of the United Democratic Front, had been involved in the unrest sweeping the country and his crime of "incitement" was too serious.

At Ulundi, in the tribal "homeland" of Kwazulu, the leader of the white liberal Progressive Federal Party, Frederik van Zyl Slabbert, and Chief Gatsha Buthelezi, the influential black moderate, made a joint appeal to hold a constitutional convention that would include all races. Although the two leaders have supported the idea of such a convention previously, the timing of today's statements suggested a possible setting of the scene for an attempt by opposition moderates to begin all-race talks of their own -- the aim being a new constitution abolishing the country's present system of white-minority rule.

Both President Botha and the imprisoned leader of the banned African National Congress, Nelson Mandela, have issued recent statements rejecting the idea of a constitutional convention.

The Johannesburg newspaper Sunday Star reported, meanwhile, that a group of unnamed businessmen were trying to arrange a meeting with leaders of the ANC at its headquarters in exile in Lusaka, Zambia.

Mpetha's imprisonment today came after judges of South Africa's highest court had recommended clemency.

Mpetha was convicted two years ago of "incitement" to violence. He was accused of making speeches that inflamed black youths during a bus boycott in Cape Town in 1980, causing them to stone two passing white motorists to death. At his trial, Mpetha denied making the speeches or being near where the motorists were killed.

He spent nearly three years in prison during his long trial with 18 other blacks, after being denied bail. Under South African law, the three years of pretrial detention will not be deducted from his sentence.

For much of the trial, Mpetha was too ill to attend court. He suffers from a diabetic condition with circulatory complications, which caused him first to have a toe amputated and then a leg. On days when he did attend the trial, he was carried into the courtroom.

When the trial ended, Judge Denys Williamson released him pending appeal on bond of one rand, about 50 U.S. cents. Mpetha lost the appeal and the bail was revoked today. His son, Oscar Mpetha Jr., said police arrived at the family home in Langa township outside Cape Town at 4 a.m. to take his father to Pollsmoor prison.

Among the UDF leaders arrested today was Steven Tshwete, regional president of the front's border region north of the port city of East London, which has become a center of unrest during the past week. Tshwete is also a member of the front's national executive, the fourth to be arrested since Friday.

Police confirmed the arrest of four other leaders but did not disclose their names or where they were arrested. Thus far, 38 leading UDF members have been charged with treason, a capital offense in South Africa. The government charges that the UDF, a loose coalition of more than 700 community organizations, is behind the unrest that has claimed more than 625 lives in the past year.

Elsewhere today, police said they fired shotguns, rubber bullets and tear gas into a crowd of blacks who had stoned a patrol in Zwelethemba township, outside Worcester in western Cape Province. Authorities also said that rioters had fire-bombed a house in New Brighton township, outside Port Elizabeth, and that a crowd of students had set fire to a school in Umlazi township, outside Durban.

In the Namibian capital of Windhoek today, 25 prominent members of the territory's major black nationalist movement, the South West African People's Organization (SWAPO), were arrested when they attended a proscribed rally in the black township of Katutura.

The rally was called to mark the anniversary of the first armed clash between SWAPO insurgents and South African forces in Namibia 25 years ago. The organization has been waging a bush war ever since then to try to force South Africa to grant independence to the disputed territory of Namibia.