An irate funeral crowd of blacks, angered by the slaying of a civil rights lawyer and inflamed by calls for retaliatory violence, pursued and stoned a black soldier to death here today, then burned his body with gasoline.

In the black township of Mamelodi, outside Pretoria, there was a similar mob slaying today, with the burning of the corpse, amid signs that black militance has reached a new pitch following five days of rioting and fighting among nonwhites that has left 60 blacks and Asians dead.

The burned body of a third black man was found near Cradock, in eastern Cape Province, police said, but no details were immediately available about his death.

Here in Wariya, a group of several hundred youths surrounded a truck carrying four black soldiers of the Ciskei homeland Army. When the driver left the vehicle, the youths chased him, pulled him down and smashed him with rocks as he lay prostrate.

When he seemed to be dead, as the crowd that had gathered around shouted "necklace, necklace" -- which seemed to be a phrase familiar to them for the grisly act about to be performed -- some of the attackers pulled a tire over the soldier's body, spread-eagled his arms and legs on either side of it, poured gasoline into the rubber casing and lit it.

The atmosphere at today's funeral, for Victoria Mxenge, 43, a slain civil rights lawyer, in the tiny village of Wariya in the tribal homeland of Ciskei, seemed more aggressive and radical than others that have been held during the unrest that has wracked South Africa for nearly a year.

For the first time there were open calls to arms and to violence. Speeches by a number of black leaders revealed a new ideological radicalism, with attacks on President Reagan, the United States and "capitalist imperialism," which were portrayed as being allies of South Africa's white-minority government and its apartheid system of segregation.

Deepened divisions in the black community were also clear in the radical attacks on the moderate Zulu leader, Chief Gatsha Buthelezi. Buthelezi, who is in Israel on a visit, was described as "an agent of the oppressors," and his Zulu-based Inkatha movement was blamed for the factional fighting in Durban and openly accused of complicity in the assassination of Mxenge.

Mxenge, a leading member of the militant United Democratic Front, died when four men attacked her with guns and a hatchet outside her home in Durban's Umlazi township Aug. 1.

The killing, which leaders of the front charge was the work of an assassination squad that has killed a number of radical black leaders recently, touched off the four days of rioting in Durban.

What has added fuel to the anger of United Democratic Front supporters is that Mxenge's husband, Griffiths Mxenge, also a civil rights lawyer, was assassinated in similar fashion four years ago.

The murder of the soldier at Mxenge's funeral today resembled the killing of a policeman at her husband's funeral here in November 1981.

On that occasion the crowd turned on a plainclothed policeman, Constable Albert Tofile, during the service and began stoning him.

In the first of what has since become a series of similar acts in such situations, Anglican Bishop Desmond M. Tutu, who later won the Nobel Peace Prize, leaped from the platform where he was delivering the eulogy and flung himself over Tofile's prostrate body to protect it from the stones.

Thinking he had saved him, Tutu returned to the rostrum, his white surplice soaked with Tofile's blood. But later the crowd dragged the constable from his car and killed him.

Today's attack came as the funeral service ended and the crowd of 10,000 mourners began crossing a highway to the village cemetery, set on a rise overlooking miles of open space with the dry grass frosted a dun color in the Southern Hemisphere winter.

As the crowd streamed across the highway, a small pickup truck, which appeared to have arrived by chance at that moment, slowed down and began threading its way through them.

Suddenly an angry buzz ran through the crowd as people noticed that the truck had Ciskei government license plates, signaling that its black occupants worked for the homeland administration set up by South Africa under the apartheid policy.

"System blacks," as such people are called by the radicals, have become primary targets during the unrest, which is aimed at disrupting the administration of black areas and rendering them ungovernable.

As the crowd swarmed around the truck, forcing it to stop, the driver, dressed in the dark green uniform of the Ciskei homeland's small Army, got out and gave a "black power" salute in what seemed to be a bid to save himself and his three companions by identifying with those in the crowd. Instead, they began hurling rocks at the truck.

In a panic, the driver started to run. With blood streaming down his face, he raced down the highway, threading his way through the crowd, then climbed through a fence and headed across a stretch of open ground.

A group of young blacks caught him and pulled him to the ground. As he lay there they smashed rocks into his skull and body. Then came the "necklace" burning.

As flames engulfed the corpse, one youth strode out from the crowd to kick the soldier's body and fling another rock at it. A second youth, waving his fists in the air, shouted at reporters: "We are going to burn them all! We must!"

The three other soldiers in the truck were hit by rocks and cut by broken glass but managed to escape by racing through the crowd in their vehicle.

The funeral speeches that preceded this incident reflected an increased militancy on the part of the radical movement following the Durban riots. Typical of the aggressive tone was a speech by Steven Tshwete, president of the local region of the United Democratic Front, who said the time for nonviolent protest was over.

"Now we are going to fight," he told the cheering crowd. "We are going to use everything we can lay our hands on, even if it is gunpowder. If we have to shoot to get our liberation, then we are going to shoot. If we have to liberate ourselves with the barrel of a gun, then this is the moment."

There was a string of assertions from the speakers at the six-hour funeral service that "we all know who killed Victoria," but they stopped short of specific accusations and blamed the white authorities in general terms.

Some speakers openly accused Inkatha of collusion, reflecting the deepening rift among the two major black movements following the bitter clashes between them in Durban.

The speeches were also characterized by several sharp attacks on the United States, something that had not been heard at the funeral rallies until now and that seemed to reflect an increasing radicalization.

Attacking Reagan for seeming to justify the South African government's declaration of a state of emergency in 36 towns and cities three weeks ago, the Rev. Smangaliso Mkwatshwa, a Roman Catholic priest who is secretary of the Southern African Bishops' Conference, said: "Botha and big business must try to support apartheid because apartheid supports their class and business interests. And Reagan wants to ensure that Yankee imperialism is protected and promoted."

Referring to the unfurling of a Soviet flag at a recent funeral rally, Tshwete said: "We were very happy to see that. We are not being oppressed by Russia; our oppression began long before the Russian revolution."

Meanwhile, a visiting U.S. federal appeals court judge, Nathaniel Jones of Cincinnati, who was arrested yesterday after entering a black township near here in violation of an emergency proclamation restricting entry to the segregated townships, said he had been told charges against him had been dropped.

Jones, who is black and is here as a representative of the International Commission of Jurists to monitor trials of dissidents, said he entered the township without knowing about the proclamation, which had been issued the day before. He said he was taken there by two white South Africans who wanted him to meet a black friend.

Police held him and his hosts for about four hours, Jones said. "They knew who I was but it made no difference. There was some very hostile behavior. I think it was an affront to the United States and to the whole notion of civility."

Jones said the attorney general of eastern Cape Province telephoned him this morning to apologize and tell him to ignore the summons that had been served on him to appear in court Monday.

In Johannesburg today, a meeting of 40 mayors from black townships around the country adopted a resolution calling on the government to release Nelson Mandela, imprisoned leader of the outlawed African National Congress, and to call a national convention of all population groups to devise a new constitution for the country.

The mayors, who are regarded as "collaborators" by the radicals and have been targets of attack during the unrest, also called on the government to make a "statement of intent" that would include the extension of South African citizenship to blacks, the establishment of a single racially integrated educational system and the repeal of all discriminatory laws.