The South African government extended its state of emergency tonight to cover Cape Town and much of the surrounding area, scene of nearly two months of continuous political unrest, following predawn police raids in which at least 66 antigovernment activists were arrested.

In Johannesburg, meanwhile, black youths rioted in the white downtown business district for the second consecutive Friday, breaking windows and looting several shops in what appeared to be a planned and well-organized assault.

The emergency's extension, which police officials long had said they hoped to avoid, coupled with the new assault on white shopping areas, indicated that South Africa was entering a new, uncertain and even harsher phase in the cycle of black unrest and official reaction.

The incident in Johannesburg led to allegations by some shop owners that foreign television crews had been tipped off beforehand and had helped instigate the violence. The allegations were denied by journalists but were repeated on the state-run television network tonight, and they are certain to add fuel to the government's campaign against foreign news media. The government contends that foreign journalists have distorted and contributed to the level of unrest in this white-ruled country.

At least six journalists were detained briefly by police today, including a correspondent for The New York Times.

The extension of the emergency is also likely to intensify international criticism of the government, which has been under strong pressure from the United States and other western trading partners to scrap the emergency measures and make meaningful changes in the system of strict racial segregation known as apartheid.

South African President Pieter W. Botha signed a proclamation adding to the emergency decree eight magisterial districts covering Cape Town, the black and mixed-race townships on its outskirts and several towns in the interior. It took effect at midnight Friday and brought to 38 the number of districts under the decree, which was declared July 20.

Law and Order Minister Louis le Grange announced the extension in a terse statement noting only that "the unrest situation in the western Cape Province and certain Boland interior areas has reached such proportions that the government has decided to declare a state of emergency."

At least 64 persons have died in the Cape Town area during the last two months following the arrest of the Rev. Allan Boesak, a mixed-race Dutch Reformed minister and civil rights leader, and the crushing by police of a peaceful demonstration he had organized to demand the release of imprisoned black nationalist Nelson Mandela.

The violence has intensified in relatively affluent mixed-race or "Colored" neighborhoods in recent days following tough new police tactics. Yesterday police pursued Colored youths through the streets of central Cape Town, scattering lunch-hour shoppers with water cannons and plastic whips.

The latest victim of violence was killed by police this morning in a black township near the town of Paarl, according to witnesses. They said a 23-year-old man died when police opened fire with birdshot at blacks protesting the tearing down of a tent sheltering about 100 black squatters.

Only yesterday, Botha announced that he was lifting the emergency in six districts in Transvaal Province and eastern Cape Province where he said unrest was easing. More than 800 persons have been killed in the 14 months since political violence first broke out in black townships.

The decree gives security forces sweeping powers, including the power to cordon off areas, declare instant curfews and arrest and hold for 14 days without charge or access to a lawyer anyone deemed a threat to state security.

But even before the decree was announced, police swooped down on members and sympathizers of the activist United Democratic Front in their biggest crackdown in Cape Town's recent history. Police confirmed 66 arrests, and there were unconfirmed reports that 14 to 19 others also were held.

Those arrested are being held under a provision of the state's broad Internal Security Act, a law almost as sweeping as the emergency decree. Almost the entire western Cape Province leadership of the front was arrested, along with teachers, clergymen, students and trade union organizers.

Last Friday's violence in the congested central area of Johannesburg appeared to be a spontaneous clash between black youths and white apartment dwellers. But today's rapid assault appeared premeditated.

Witnesses said two groups of about 20 youths, armed with plastic shopping bags filled with rocks and bricks, rushed into the shopping district early this afternoon smashing windows and raiding about 20 shops.

The assault was over within minutes, and the youths vanished before police arrived and before pistol-wielding shop owners could retaliate.

One of the white shopkeepers, identified in local press accounts as G. Rayson, said two camera crews, one from the British Broadcasting Corp. and the other from NBC News, had appeared on the scene before the rampage began. He charged that the journalists may have "instigated the whole thing."

Police officials in recent weeks have accused television crews of encouraging rioting for their cameras and have warned that they would crack down on journalists they believed were portraying the security situation in South Africa unfairly. An official spokesman said tonight that police would investigate the new allegations.

Keith Miller, an NBC correspondent who was with one of the crews on the scene, said his crew had received no tip-off about today's incident, nor had it encouraged the violence. Miller said the crew had been driving around downtown Johannesburg anticipating a renewal of last Friday's trouble but arrived after the rampage had started.

"We never did get a picture of anyone doing anything except sweeping up broken glass," he said.

A two-man CBS crew was held briefly for questioning by police this afternoon. Outside Cape Town, another two-man CBS crew, along with a free-lance photographer and Sheila Rule, a correspondent for The New York Times, were detained for several hours by police after being picked up on a college campus in a Colored township following a confrontation between police and students. They were released without charge.