The peaceful transition to black-majority rule in neighboring Rhodesia has generated new pressures here in South Africa for political reforms that are focusing on a drive to free imprisoned black nationalist leader Nelson Mandela.

A full decade before Robert Mugabe started his guerrilla struggle in Rhodesia, Mandela began preparing for active struggle against South Africa's white-minority government and has been languishing ever since at the bleak penal colony for political offenders on Robben Island off Cape Town.

Since Mugabe's sweeping victory in Rhodesia's election last month, more than 42,000 persons here have signed petitions calling for the release of Mandela, 61. A lawyer by training and member of Tembu royalty, Mandela had pleaded innocent to sabotage charges for which he was given a life sentence in 1964.

The drive for his release was started by the Johannesburg Post, whose black editor Percey Qoboza wrote that Mandela, who "stands head and shoulders above all our other leaders," should be freed to begin "a serious era of negotiation that will bring peace to our land."

Otherwise, Qoboza wrote, South Africa will face internal strife that would make the seven-year-long Rhodesian civil war "look like a tea party."

Although the drive was not expected to produce any immediate results, it says much about the present state of black and white politics in this country. Significantly, it has drawn attention from certain white groups that a few years ago might have ignored it.

A spokesman for the South African Foundation, an influential business lobby, said the government should be sensitive to requests for Mandela's release. Prime Minister Pieter Botha was booed and hissed last week when students at the University of Stellenbosch, South Africa's most prestigeous Afrikaner school, challenged the continued detention of Mandela and were told by Botha that the black leader was an "arch Marxist" supporting violent revolution.

Less surprising has been support for the "free Mandela" campaign from hundreds of white students at the more liberal English-speaking Witwatersrand University and by the South African Council of Churches, whose membership is multiracial. Opposition parliamentary leader Frederick Van Zyl Slabbert said he is ready to deal at a national convention with Mandela if the black nationalist disavows violence.

Among blacks, response to the campaign has highlighted the fact that, despite attempts by the government to obliterate Mandela's influence by a ban on printing his picture and anything he has ever said, he continues to occupy a unique notch in their minds. As the former president of South Africa's most enduring nationalist organization, the banned African National Congress, Mandela is still revered among many blacks as their genuine reader, a hero and martyr to their emancipation.

Even rival black groups that seldom see eye to eye on anything regard Mandela as the one man whose presence is essential before any meaningful exchange can take place with whites on a more equitable constitutional deal for this country.

Therein lies Mandela's power.

"because of his age, he is a symbol; he can unite us," said black minister Allen Boesack, who regards the campaign as significant for the common ground of action it has offered the often hostile movements of the so-called black consciousness leaders on the one hand and Zulu Chief Gatsha Buthelezi on the other.

Buthelezi told a rally in Soweto today that he considers Mandela as one of the major black leaders of South Africa and that "if the people choose him as a leader I would be prepared to serve him if that was the mandate of the people."

Mandela's unifying ability among blacks as well as the uproar his release would precipitate from the already troublesome right wing of Botha's National Party both help explain why the prime minister made it clear at Stellenbosch that he would not consider releasing the black leader.

But the government's refusal to deal with Mandela illustrates the chasm that persists between those it perceives as credible black leaders and those that blacks regard as their leaders.

"there is a tendency to negotiate with leadership which is acceptable to you but it's a paper tiger sort of leadership and it doesn't represent the true sentiments of the masses of people," Qoboza said. "if they have learned a lesson from Zimbabwe, they will let him Mandela out and talk to him."

"if the government is sincere about a new dispensation it must provide concrete evidence that is is prepared to take black opinion and authentic black leadership seriously," said Peter Storey, vice president of the South African Council of Churches. "the release of Mandela after nearly 16 years in prison would provide such evidence."

The campaign therefore has helped set the stage on which Botha plans to discuss the future role of blacks in this country. The discussion will proceed, however, without the participation of at least one important black leader regarded as necessary for its success by a wide spectrum of blacks. Furthermore, it will be carried out within the context of South Africa's present security legislation that has silenced most black political activitists and dissidents.

For this reason, some black spokesmen say there can be no talks such as those in London last year that led to a black-majority government in Rhodesia. There the black nationalists had the ongoing guerrilla war as a bargaining chip.

At the moment, many blacks believe that the only way to increase their bargaining position against the government is through armed warfare and calls for Mandela's release come at a time of growing support for his African National Congress, now waging a sporadic urban guerrilla war against Pretoria.

On April 4 a group of the organization's guerrillas sent two rocket-propelled grenades into a police station 10 minutes with hand grenades and in Johannesburg and barraged it for machine-gun fire. It was the fourth attack on a police station in less than a year.