The Rhodesian national election held last week received its most important endorsement today when a British Commonwealth observer team announced it had found the process to be free and fair.

The 11-member team criticized Britain for a "lack of a sufficient degree of impartiality" toward all the parties contesting the election and some "deficiencies in implementation" of the peace agreement signed last December in London. But its conclusion about the fairness of the election has given London important support for acceptance of the results by the Commonwealth and the rest of the international community.

In Washington, Secretary of State Cyrus Vance congratulated Britain on successful completion of the election.

["We urge the forces on all sides to exercise restraint during this historic moment. Any attempt to abort the political process would be condemned by the world community," he said. "We therefore urge the parties to the process in Zimbabwe and all nations to respect and support the electoral outcome."]

With the vote counting set for Monday, there is widespread speculation among observers from many countries that Robert Mugabe may have garnered an even larger percentage of the vote than expected.

Interest is centered on which leader British Governor Lord Soames will ask to form the first independent government. The white Rhodesian establishment and the British administration here have privately indicated a preference for a coalition of parties that would exclude the Marxist Mugabe.

Mugabe, however, declared today that even if his party won only 35 of the 80 black seats in the 100-member National Assembly, Soames was "duty-bound" to call on him first to form a government.

"He has to call upon me first," said Mugabe, adding later that he hoped his former guerrilla partner, Joshua Nkomo, would join him in a coalition government. Mugabe said he has asked Nkomo to do so but had not yet gotten an answer.

Indications that Mugabe may have topped the voting results came today in statements from various parties. While Mugabe told reporters he thought the amount of intimidation surrounding the election "had no effect on the results," officials of three other parties, including Nkomo's Patriotic Front and Bishop Abel Muzorewa's United African National Congress, told newspapers they would not commit themselves yet to accepting the results because of the widespread intimidation.

In an indication of the delicate position Mugabe faces even if he does win the election, he also said he would like Soames to stay after the installation of a new black-majority government. He cited two reasons: to give confidence to the whites that the black-run government will not make "mincemeat" out of them and to act as a deterrent against any attempt to stage a white-led coup.

Mugabe's party has been keenly aware of hard-time opposition within a section of the white establishment, in particular in the white-led armed forces. Their apprehensions were only partly assuaged by a secret meeting last week between Mugabe and the military commander, Lt. Gen. Peter Walls. According to Mugabe, Walls pledged that he would not support military action to stop a legally elected government.

A hasty departure by Soames would mean the British official would leave behind "a situation of potential danger," Mugabe said today. He had returned from a four-day trip to neighboring Mozambique for talks with President Samora Machel, his strongest backer in the seven years guerrilla war here.

The interim report by 33 observers from 11 Commonwealth countries noted with satisfaction that despite a highly armed population in Rhodesia, there was less violence than had been feared. It said that despite intimidation during the campaign, parties had "adequate opportunity" to get their views across.

Stating that it would be "misleading to apply conventional yardsticks in assessing" a unique situation, the group said, "the election up to the end of polling can be considered to have been free and fair to extent that it provided an adequate and acceptable means of determining the wishes of the people in a democratic manner."