Ten thousand supporters of South Africa's white extremist movements rallied peacefully today, marking the 25th anniversary of the Republic of South Africa at the foot of a monument to the Afrikaner pioneers on a hill outside Pretoria.

In three hours of fiery speeches, far-right leaders called on Afrikaners to fight to defend white control of South Africa. There was none of the violence, however, that has accompanied other recent far right-wing demonstrations. The government gave permission for the rally to be held and the police stayed away.

In Johannesburg, 30 miles away, police intervened for the third time this week to break up an antiapartheid meeting at Witwatersrand University. Forty-eight students and teaching staff were arrested there on Thursday and Friday and are spending the weekend in police cells.

Although today's rally at the monument was the biggest that has been held by the extremists, whose growth has caused concern in the government of President Pieter W. Botha, it was smaller than expected and observers said they felt it indicated a limit to the far right's following.

Botha's National Party government, which has been under attack by the extremists for initiating limited reforms of the apartheid system of segregation, initially said it would ban the meeting. It later reversed that stand, averting a possible show-down confrontation by saying it had been assured that the rally would not be political.

After their success two weeks ago in violently disrupting a National Party rally in Pietersburg, the white supremacy extremists had named today as the occasion when they would demonstrate their burgeoning strength at the Afrikaner community's most sacred shrine.

Supporters came in automobiles and buses from all parts of Transvaal Province, the regional stronghold of the far right. But they managed to fill only one-third of the big open-air amphitheater that fans out on the hillside below a heavy, somber-looking monument building that resembles a mausoleum.

The rally was organized by the four major right-wing movements: the Conservative Party, the more extreme Herstigte National Party, the neo-Nazi Afrikaner Weerstandsbeweging (Afrikaner Resistance Movement) and an umbrella organization called the Afrikaner Volkswag, or "sentinels of the Afrikaner people.".

The Afrikaner Resistance Movement's leader, Eugene Terre'Blanche, got the most enthusiastic round of applause when supporters escorted him into the amphitheater beneath a huge red, white and black swastika-like banner.

"We are a blessed people living in a promised land," Terre'Blanche told the crowd. "We cannot give away this land because it is not ours to give. It is sacred land and it cannot be used as a gift of appeasement by politicians who do not feel themselves bound by that sacred trust," he added in oblique reference to Botha.

At Witwatersrand University, a large contingent of police forced about 1,000 black and liberal white students to abandon their meeting on segregated education.

Student leaders negotiated with the police, who eventually agreed to withdraw if the students left.