When the polls closed yesterday, almost 2.7 million blacks had voted in Rhodesia's British-supervised national elections, bringing the participation of eligible voters to an extraordinary 93.6 percent.

The high turnout of 2,699,450 voters tops the 64 percent participation in last year's elections that were boycotted by two guerrilla movements and that failed to get international recognition. The larger vote this year gives some indication of the effectiveness of the guerillas' efforts last year to keep people from voting in what they regarded as a sham election. White Rhodesians voted earlier to select their members of Parliament.

Meanwhile, an American observer team has concluded that the three-day elections were "free but not entirely fair."

The four-man delegation from the New York-based Freedom House, headed by civil rights leader Bayard Rustin, praised the British governor and the Rhodesian civil service for providing an "essentially free" electoral process. However, the "climate of fear" and intimidation in which some parties had to campaign reduced the fairness of the election, their report said.

The observer group criticized the successful efforts of "formerly externally-based parties," meaning the guerrilla organizations "in maintaining areas in which the messages of other parties could not be effectively presented."

Nevertheless, the election "may be judged a further step toward majority rule," the report said. Freedom House is a non-governmental agency dedicated, in its words, to strengthening "free institutions around the world." d