Vice President Mondale will tell Prime Minister John Vorster next week that the United States has adopted a new and tougher policy of opposition to South Africa's racial practices, administration officials said yesterday.

The essence of the new policy is that the United States will oppose apartheid in deed as well as philosophically, reporters were told in a briefing on the 10-day overseas trip, which Mondale is to begin today.

The officials, who cannot be identified under the ground rules of the briefing, declined to say what concrete steps the United States is prepared to take. However, they did not disavow a list of potential steps reportedly discussed by U.N. Ambassador Andrew Young earlier this week with U.S. ambassadors in Africa. These include the withdrawal of the U.S. military attache from South Africa, tightening up visa requirements for South Africans, severing links between U.S. and South African intelligence and curtailing Export-Import Bank credits to the Pretoria government.

The policy which Mondale will spell out to Vorster in their meeting Thursday in Vienna is the result of discussions over the past six weeks involving President Carter, Mondale, Secretary of State Cyrus R. Vance, Young and other senior diplomats, officials said.

The change in official statements about South Africa began in major fashion with then Secretary of State Henry A. Kissinger's declarations in Lusaka, Zambia, on April 27 last year. Kissinger declared then that the United States will work for "a peaceful end to institutionalized inequality" in South Africa and appealed to South Africa to move toward "equality of opportunity and basic human rights for all South Africans . . . within a reasonable time."

Kissinger was reluctant to press South Africa on its internal policies, however, because that country was seen to be the key to a transition to black rule in neighboring Rhodesia as well as Namibia, which has been run by Pretoria as a colony.

The Carter administration policy is to elevate U.S. concern about apartheid within South Africa to the same level as its concern about Rhodesia and Namibia, according to officials involved. Their belief is that South Africa will still cooperate in the majority rule transition in the other two areas because it is in its national interest to do so.

On the Mondale trip next week "we are going to have to make it clear that we do not find policies they [South Africans] are pursuing internally with regard to a large part of their population acceptable," an official briefer said. "It is our conviction there has to be full political participation by all sectors of their society."

If South Africa will chart a new and positive course, it will have U.S. support and understanding. But "unless there is some basic change, our two governments are simply going to be going in different directions [with] some very important implications," the official said. He did not elaborate.

Another policymaker added that if South Africa fails to change its internal policies, it should be clear "that they cannot count on the U.S then intervening in their behalf . . . if the situation becomes much worse down the road."

Reporters were told that Mondale will not present Vorster with a U.S. plan to follow, but will explain as clearly and as firmly as possible the U.S. policy direction and its implications.

Earlier this week, Britain and the United States announced a joint iniitative aimed as a renewal of negotations between white-ruled Rhodesia and the liberation groups and "front line" African states on its borders. The negotiations will be pursued by British and U.S. diplomats operating rom a headquarters in one of the black African states.