CAPE TOWN, South Africa -- Tens of thousands of black activists are waging a concerted protest against South Africa's white authorities in a campaign that has shaken assumptions about gradual reform and Friday brought the brief arrest of this country's most prominent critic of apartheid, Anglican Archbishop Desmond M. Tutu.

Tutu and his wife, Leah, were among hundreds arrested countrywide. They were hauled away for protesting police beatings of clerics at a Cape Town demonstration earlier in the day. They and 34 other marchers were held three hours and released. No charges were filed but police said they are considering charges of holding an illegal march.

In Washington, State Department spokeswoman Margaret Tutwiler demanded a full report from Pretoria and urged the government "to permit the peaceful expression of political dissent."

While Tutu and other prominent church leaders and activists were making highly visible protests, Hilda Ndude, a young working mother of five took on government forces a few nights ago simply by waiting until after 7 o'clock to return to her home in nearby Guguletu.

Broke 'Restriction Order'

The police hauled Ndude into the Guguletu station and charged her with breaking her "restriction order," which puts her and about 500 other political activists under nighttime house arrest.

Two nights later, Ndude did it again. This time she and her children spent the night in jail, sleeping on the floor.

"When people are fighting injustice, I have to be part and parcel," Ndude, now out on bail, said the other day, violating the restriction order again by granting an interview.

"I take into account that I am a bread-winner," she added as she snapped a pair of overalls onto her 4-year-old daughter. "But if I take a back seat, who's going to fight for us?"

For nearly a month, Ndude and the other activists have used the principles of civil disobedience to tap what has turned out to be a brimming reservoir of militancy among the disfranchised black majority.

Police have whipped, tear-gassed and arrested activists for marching and singing freedom songs in Cape Town streets, for trying to board whites-only buses in Pretoria, for picnicking at whites-only beaches, for asking for education at whites-only schools, for picketing the visit of an international rugby team, for holding outdoor rallies and for openly defying their restriction orders.

Undermines Assertions

The defiance campaign, the largest anti-apartheid protest in more than three years, has begun to undermine, at home and abroad, government assertions that "apartheid is dead" and cast doubt on new acting President Frederik W. de Klerk's ability to negotiate peaceful change in South Africa.

"You cannot expect to have productive negotiations with the same people you sjamboked (whipped) and tear-gassed the previous day," says Jan van Eck, who monitors the campaign for the liberal white Democratic Party.

The most violent clashes with police have occurred in Guguletu and other townships near Cape Town, where youths attacked by police have abandoned passive resistance and thrown rocks at police vehicles.

Tutu has criticized the authorities' "incredible impatience and eagerness to use the full range of their arsenal."

"It's going to be a miracle if many of our children are not killed," said Tutu, the 1984 Nobel Peace laureate. A policeman and a truck driver have died in incidents related to the defiance campaign.

Spotlights Segregation

The campaign, organized by a loose affiliation of anti-apartheid groups under the banner of the Mass Democratic Movement, was designed as a way to embarrass the government in the weeks before next Wednesday's general elections by calling attention to the remaining vestiges of racial segregation and repressive emergency laws that prohibit even peaceful anti-government protests.