South African government embarked today on a massive crackdown on dissent against its racial policies, banning 13 civil rights groups, shutting down two major black newspapers and arresting scores of black and white antiapartheid leaders.

The sweeping move came unexpectedly in pre dawn police action that reflected a concerted bid by the white government to put an end to an up surge of black, dissent over the past 13 months.

By resorting to force, the government has supressed practically over night all moderate black groups that advocated peaceful changes in South Africa's system of apartheid, or strict racial segregation.

Prime Minister John Vorsters decision may have come in the heat of election campaign - apparently designed to court the conservative section of the electorate - but observers here said his action in the long term could make racial conflict more likely and peaceful changes more difficult.

"It is a said and terrible day for South Africa," Beyers Naude, director of the now-banned Christian Institute said. "It can only hasten the end of the present regime."

Raids, arrests and other legal actions, which began before daylight, continued throughout the day.

In the process, some 70 civil rights leaders and activists were detained and confined to their homes and South Africa's leading black newspaper, The World, was closed down.

It was rated the government's biggest effort to combat racial unrest since 1960, following the Sharpville riots when 67 blacks were killed in the first big confrontation with the minority white government.

Black militancy has been building in recent months, especially since the mysterious death last month during detention of black activist Steve Biko, and observers here said the new crackdown is likely to drive black activists underground.

In today's coordinated action in most of the nation's big cities, police struck at the most prominent of the "black power" organizations: the Black People Convention and the Soweto Students Representative Council, which has spearheaded a boycott of black schools because of their inferior facilities.

Also among the banned organizations are the multiracial Christian Institute of South Africa; the leading students' groups, the South African Students' Organization and the South African Students' Movement; the Black Parents Federation; the Black Womens Federation; the Black Community programs; Union of Black Journalists; the Medupe Writers Association, and the Zimele Trust Fund.

Among those arrested were some of the current leaders of black protest groups, including Nthato Motlana, chairman of the Soweto Committee of Ten, which is seeking self-government for all-black cities such as Soweto with its population of more than a million people.

Percy Qoboza, editor of The World, also was arrested.

A few prominent whites who sympathize with black campaigns for social and economic equality were banned - the legal term here for house arrest.

The most notable victim of banning was Donald Woods, outspoken anti-government editor of the East London Daily Dispatch. He has been the most relentless critic of the government for more than a decade, and writes a synicated weekly column read by half a million South Africans.

Woods was scheduled to arrive in the United States on Friday, but now he will not be able to make the trip nor will be able to write or speak in public for five years.

Also banned was Beyers Naude, a one-time pastor of the conservative Dutch Reformed Church who left the fold in 1960 and is now the leader of the Christian Institute, a multi-racial, non-denominational group of liberal churchmen.

Naude was also banned for five years, meaning in his case that he cannot have contacts with anyone outside his family for that period.

The crackdown led to immediate protest, with even editors of pro-government newspapers saying that they feared that closing down The World was the start of a campaign to end press freddom in South Africa.

There was an abortive attempt by Johanesburg white university students to stage a protest march, but armed police turned them back within a few miles of their downtown campus. To night, however, police arrested 65 white university students as they marched to a post office intending to send a protest telegram to Kruger.

In Soweto, primary school children, who had not previously participated in the school boycott that has closed 40 high schools in the black township, began streaming out of classes. Countrywide, almost 200 000 balck students are staying away from school in the most serious non-vilent definance to the government yet staged by anti-apartheid groups.

The government actin was defended by police and Justice Minister Jimmy Kruger. He said that the various groups were outlawed and The World shut down because they "endanagered the maintenance of public order."

Kruger accused anti-apartheid militants of seeking to create "a revolutionary climate" in South Africa and confrontation between whites and blacks.

In addition to banning The World, the government also shut down its weekend paper, Weekend World, and the magazine of the Christian Institute, Pro Veritate.

Among those arrested today were Hlaku Rachidi, head of the Black People's Convention; the Rev. Smangaliso Mkhatshwa, acting secretary general of the Southern African Catholic Bishops' Conference; Shun Chetty, a former defense attorney for several black consciousness leaders, and Dr. Francis Wilson, head of economic research at the University of Cape Town.

During the early morning raids, police confiscated papers, documents and files.

"I would have liked it not to be forced upon us," Connie Mulder, minister of infromation, said. "Being in Africa and belonging to Africa, the situation is that Africa believes in strong government. When the government must show its hand, it must be just in its actions but not hesitate to act strongly."

Today's massive repression of black activists against racial descrimination is bound to be greated with universal condemnation and complicate South Africa's foreign relations, especially with the United States.

Reaction throughout South Africa itself was swift and unfavorable.

Leading opposition politicians condemned the action, as did colored (mixed-race) and Indian leaders.

Rene de Villiers, a former newspaper editor and now a member of Parliament, called it a "disastrous move, the beginning of the end of press freedom in South Africa."

Ray Swart, national chairman of the opposition Progressive Federal Party, called it "further evidence of the state of crisis that has been reached in South Africa."

"Past experience has shown that banning organizations achieves nothing because one cannot ban ideas," he said.

Chief Gatsha Buthelezi, administrative head of the Zulu tribe, said he was shocked and added:

"I would have thought it was in the government's interests to maintain at least asemblance of democracy, but it just shows how beleaguered by blacks the government must feel itself to act so rashly and unimaginatively."