The U.N. Security Council is expected to meet Friday to set up a United Nations peacekeeping force for Nambia (Southwest Africa) that will precide ever that territory's transition to independence from South Africa, diplomatic sources said last night.

Secretary General Kurt Waldheim had called last week for dispatch of 7,500 troops and 1,200 civilian officials to Namibia in what would be the largest U.N. operation since the United Nations sent a force to restore order to the former Belgian Congo, now Zhire, in the early 1960s.

The proposal, which has been expected to win quick Security Council approval was delayed when South African Foreign Minister Pik Botha and Sam Nujoma, leader of the Namibian guerrilla organization that has been attempting to gala control of territory, flew to New York to voice objections.

The five Western powers that sponsored the basic formula for Namibian independence - accepted by both South Africa and the guerrilla in July - have been consulting with both sides this week in an effort to overcome their objections to Waldheim's proposal.

Botha gave these Western efforts a new urgency yesterday by threatening to leave New York and return home.

While an informed South African official said last night there was still a good chance Botha would leave, other diplomatic sources said they had been informed privately that Botha would remain if the Security Council intended to act on the plan Friday.

The diplomatic sources said it now appeared that the council would meet to approve the plan Friday, or Saturday at the latest.

"We can't let either party leave and delay this thing further," one source said.

While Botha sent a 20-page letter to Waldheim yesterday sharply criticizing elements of his proposal, an informed diplomatic source suggested that his remarks were largely intended to convince South Africans that "he's not being a pushover."

Botha was particularly critical of the planned size of the U.N. peace-keeping force, contending that the Western powers had indicated during year-long negotiations that they were thinking in terms of a U.N. troop presence ranging from "a few hundred" to 2,000.

Diplomatic sources suggested however, that South African objections to a 7,500-man U.N. force could probably be met by stating in the resolution that the "7,500 figure would be a ceiling, and only as many would be deployed as necessaary.

The major objection to the plan voiced this week by the Namibian guerrillas and black African nations has been the lack of any provision in the Waldhelm proposal regarding registration of voters in Namibia, sources said.

The question of who is allowed to vote will ultimately decide whether Nujoma's forces or candidates regarded more favorably by South Africa win control of the new nation's constituent assembly. Black African countries therefore have expressed considerable concern over this omission.

Diplomatic sources said the frontline black nations were proving "flexible, however, in how that concern is dealt with."

"We definitely think all these problems can be worked out," one source said. "And we hope they can be worked out by Friday."