PORT ELIZABETH, SOUTH AFRICA, NOV. 28 -- The South African government today prevented freed African National Congress leader Govan Mbeki from making a speech that antiapartheid campaigners had hoped would begin a process leading to the legalization of the outlawed ANC.

A provincial Supreme Court justice refused to overrule a police commissioner's ban on Mbeki's planned rally at a soccer stadium here, saying it would have fanned the revolutionary atmosphere.

It would have been the first political rally in South Africa to be addressed by an African National Congress leader in 27 years.

Organizers of the rally said they had hoped it would start a process in which imprisoned ANC leader Nelson Mandela eventually could address the country's 26 million blacks, an event that would effectively mean the legalization of the black nationalist organization.

A district magistrate had given permission for the rally and speech, but the police overruled him, saying that it could lead to new disorders.

The chief of the security police, Lt. Gen. Johann van der Merwe, said that the rally could have had the effect of prolonging South Africa's strict state of emergency.

"There is no doubt that Mr. Mbeki is being instructed and manipulated by the ANC," van der Merwe said.

Security forces sealed off the stadium in the black township of Zwinde to prevent thousands of Mbeki supporters, scores of journalists and a dozen foreign diplomats from hearing the 77-year-old former national ANC chairman, who with Mandela was sentenced to life imprisonment in 1964 for sabotage and treason.

Mbeki was released Nov. 6 in a move that has been widely interpreted as a prelude to the release of Mandela and other well-known security prisoners. Senior government officials have suggested that reaction among blacks to Mbeki's release will determine whether Mandela and other ANC leaders are released.

The motive behind their strategy is believed to be to press credible black leaders like Zulu Chief Mangosuthu Gatsha Buthelezi to negotiate a power-sharing arrangement with the white minority. Buthelezi and most other black leaders have said they cannot join negotiations while Mandela and other security prisoners are still incarcerated.

The speech Mbeki had planned to give, which he read to reporters at a news conference here after the rally had been banned, appeared to contain nothing that would have justified the authorities' fears of a rekindled revolutionary climate in South Africa.

He did not endorse an ANC armed struggle in South Africa, or even explicitly call for the overthrow of the Pretoria government.

Instead, he made a moderately phrased appeal to blacks to dedicate themselves to a "democratic, nonracial and unfragmented South Africa."

Mbeki did not renounce his links to the ANC or the South African Communist Party, which also is outlawed, but said his 23 years in prison was "compensated by the fulfillment of knowing that I was pursuing a cause, experiencing loyalties to my ideals. I make no apologies for it. I did what I had to do."

Mbeki said he and other security prisoners suffered because they were in "basic conflict with the basic structures of the very strange society in which we live."

While he celebrated his own release, Mbeki said, "True freedom can come to all of us -- black and white, the oppressor and the oppressed -- only when the sun sets finally on the deformity of apartheid."

He added, "My incarceration and my release would have little meaning if this long nightmare of apartheid and injustice and alienation of brother from brother is allowed to perpetuate itself."