U.S. officials predicted yesteday that Washington would move quickly to open relations with the new Rhodesian government and extend several million dollars in aid.

But they conceded that efforts to help the avowedly Marxist Robert Mugabe might run into opposition in Congress.

Despite Mugabe's Marxist ideology, State Department officials portrayed the election as a victory for the West since it proved that free elections are possible in that troubled land. Mugabe was portrayed as a bright well-organized leader and the officials said they expected him to follow a pragmatic course.

This would probably mean a realistic attitude toward dealing with South Africa, which is economically important to Zimbabwe.

In Britain, the government hailed the success of the Rhodesian elections and parliamentarians of all parties praised Mugabe for pledging to work for peace and reconciliation in Rhodesia. However, the prices of Rhodesian government bonds tumbled and shares of firms with interests in Rhodesia also fell.

South African Prime Minister Pieter Botha said "the result of the election in Rhodesia is a decision by the people of Rhodesia. They will have to work it out for themselves and live with it."

He added, however, that "any neighbor . . . which allows its territory to be used for attacks on or the undermining of South Africa and its security, will have to face the full force of the republic's strength."

The official Soviet news agency Tass urged Mugabe to join forces with his former ally Joshua Nkomo, who was supported by the Soviet Union during the guerrilla war, in order to ward off any possible threat from South Africa.

Frontline African presidents Julius Nyerere of Tanzania and Kenneth Kaunda of Zambia both welcomed Mugabe's victory. Kaunda said Zambia would accept any government formed by the former guerrilla leader. Zambia was the main base for Nkomo's forces during the seven-year Rhodesian war.

In Maputo, the capital of Mozambique, where Mugabe's forces were based during the fighting, there were spontaneous victory celebrations. A government communique warned outside parties to abide by the election result.

Tanzania's Nyerere had predicted that the British would rig the elections to prevent the guerrilla leaders from winning a majority.

"This is not the first time I have been proved wrong and it is not the first time that I'm very pleased that I am wrong," he said.