About 50 persons, including Del. Walter Fauntroy (D-D.C.), demonstrated in front of the International Monetary Fund building at 700 19th St. NW yesterday to protest the fund's impending vote, taken later, to lend $1.07 billion to South Africa.

Fauntroy said he decided to participate in the protest after he failed to persuade Reagan administration and fund officials to delay or oppose the vote. South Africa's "human rights and apartheid systems must be changed," he said. "Such a regime should not benefit from the fund."

Randall Robinson, president of Transafrica, a lobby devoted to African and Caribbean issues, termed the loan "an indirect subsidy of South Africa's military campaigns."

South Africa requested the loan to pay foreign debts and help balance its economy, said Graham Newman, senior external affairs officer of the fund. Robinson said the loan was sought because of falling prices of the country's chief exports -- gold and diamonds -- coupled with increased military spending.

The vote by the 22 executive directors of the 140 member-nation fund granted the first loan to South Africa since 1975, when the country received $453 million.

The loan has drawn criticism from at least 35 members of Congress, said Fauntroy, who said Treasury Secretary Donald T. Regan had been asked to oppose or delay the vote, as well as from the Congressional Black Caucus and other groups.