A U.N. conference called to implement independence for Namibia collapsed tonight when South Africa, which administers the territory, said it was "premature" to fix a date for a cease-fire and elections.

The South African statement came after the leader of the internal government established by South Africa in Namibia in definace of the Nited Nations had said more time was needed to assure impartiality of the United Nations, which is to supervise the elections, and to gain assurances that a democratic form of government would be established in the territory.

A spokesman for the South-West Africa People's Organization, which has led the guerrilla war for independence, immediately announced that it would demand U.N. Security Council action to impose "comprfehensive, mandatory sanctins, including an oil embargo" against South Africa.

That will pose an early test of the Africa policy of the incoming administration of Presiden-elect Ronald Reagan. There has been speculation that South Africa may have been relying on an American veto in the Security Council under the new administration to protect Pretoria from sanctions.

South Africa said it acted because requirements for establishing trust and confidence in the United Nations had not been fulfilled.

The rejection came as a surprise because:

In October, the South African government had been secretly assured by the United Nations that formal U.N. action would clarify the troubling issues in connection with trust and confidence, according to Western sources.

Earlier in the meeting here, South Africa had been secretly informed that SWAPO was prepared to issue a declaration reassuring the people of Namibia on the question of democracy under an independent government, according to informed diplomats.

However, Western sources reported that Dirk F. Mudge, chairman of the Democratic Trunhalle Alliance that heads the interim internal government in Namibia, Sunday told representatives of the five Western nations sponsoring the independence plan that he would not agree to implementation until the internal parties were assured of at least a 50-50 chance of defeating SWAPO in the proposed election for a constituent assembly.

Mudge said SWAPO would turn Namibia into a one-party, totalitarian state.

The collapse of this conference means that independence will be postponed at least until next year. Under the U.N. plan, the cease-fire in the guerrilla war was to take effect in March with elections in October under U.N. peacekeeping force.

In a press conference today, Mudge said he could not say how much time would be required to satisfy the conditions he had set forth. There were unconfirmed rumors that he may have said 18 months to two years in conversations with some diplomats.

"I do not rfegard the U.N. plan as dead," Brian Urquhart, U.N. undersecretary for special political affairs and conference chairman, told reporters earlier in the day.

Urquhart said it was now up to the governments concerned to decide what action to try next, emphasizing the global risks involved in continuation of the fighting.

At least some of the leaders of the internal parties set up with the approval of South Africa as rivals to SWAPO indicated their full agreement with Mudge's statement, insisting that "the ball is now in the United Nations court."

"Our aim now is to find ways to keep the whole independence process alive by keeping lines of communication open between the parties concerned," U.N. spokesman Francois Giuliani said. o

A spokesman for SWAPO, which had remained largely silent in the proceedings, was sharply critical of the outcome. More will be heard at the final session Wednesday when Sam Nujoma, head of SWAPO, is scheduled to speak.