The South African government has disclosed a two-pronged policy for trying to end a long war of attrition that it has waged against thousands of black people living in shantytowns outside Cape Town.

A week ago, President Pieter W. Botha announced the scrapping of a 20-year-old policy that was designed to exclude blacks from a large area around the city.

Today, in a related move which is certain to cause controversy, the new minister in charge of black affairs, Gerrit N. Viljoen, said that while the estimated 70,000 shanty dwellers in the area could stay, they would have to be resettled at another spot farther from the city where they can be "better controlled."

Among those who must move are the residents of a shantytown called Crossroads. The government gave the settlement a reprieve in 1979 after its attempts to demolish shanties there caused an international outcry.

The twin moves are seen here as an attempt at cautious reform by the Botha administration, aimed at easing the restrictions on blacks living and working in the area while tightening controls over where they may live.

Critics of the government's segregationist policy of apartheid welcomed Botha's announcement Sept. 24 that the old policy would be scrapped. But they expressed fears today that the people of Crossroads would resist any renewed attempt to move them and that this could lead to trouble in the shantytown.

Helen Suzman, a member of Parliament and a leading civil rights advocate, commented after Botha's announcement last week, "It's a great bit of news, but it's a pity that it has come years too late, after so much misery and insecurity for the people concerned."

After Viljoen's announcement today, Suzman said that it would be "totally mad" to attempt any mass removal of the 70,000 people in the shantytowns.

"They must only be moved after a process of patient negotiation with individual families. Otherwise there is going to be trouble," she warned.

Timo Bezuidenhout, the official in charge of black administration in the Cape Town area, said in a telephone interview that the removal of the shanty dwellers would not begin until next year, and that "every effort" would be made to persuade them to move voluntarily.

Asked what would happen if they did not agree to the move, Bezuidenhout, who has been the overseer of many raids on the shanty dwellers, replied, "I really don't know. The government would have to decide what action to take. But I am sure they will want to move."

Botha announced the scrapping of the old policy just 10 days after his inauguration as the country's first executive president under a new constitution that grants limited political rights to the mixed-race and Asian minorities for the first time but that still excludes blacks.