The Rev. Jerry Falwell, leading a public-relations campaign on behalf of the South African government, called Nobel peace laureate Bishop Desmond Tutu a "phony" yesterday for presuming to speak for nonwhites in the racially separated nation.

Falwell, leader of the Moral Majority, has just returned from a 5 1/2-day tour of South Africa and is urging increased outside investment there. He contended that nonwhites with whom he spoke, both national leaders and people on the street, told him that disinvestment in South Africa would hurt people rather than the government.

At a news conference here, Falwell repeated his charge that Tutu is a "phony."

"If he maintains that he speaks for nonwhites, he is a phony," Falwell said. "I'm not referring to his religious experience."

He quoted the black mayor of Soweto as saying of Tutu, "We did not elect him. The people respect him as a man of God, but he does not speak for us."

The Rev. Joseph E. Lowery, president of the Southern Christian Leadership Conference, denounced Falwell's charge against Tutu as "an old trick in the hands of a new magician."

"That's an old George Wallace, an old Bull Connor segregationist tactic to discredit civil rights leaders," Lowery said. "Segregationists always manage to find some blacks who will say what they want. One distinguished Methodist leader there said he had no idea what he Falwell was talking about."

Lowery scoffed at Falwell's contention that disinvestment in South Africa would cause the "little children to starve."

He said disinvestment "might cause some temporary inconvenience" but that it was necessary before the country could get on with long-term economic development. "Disinvestment may hurt blacks but not nearly as much as dehumanization," he said. "He's Falwell identified with the immoral minority there."

House Budget Committee Chairman William H. Gray III (D-Pa.), sponsor of antiapartheid legislation, said, "As a Baptist minister, I am deeply ashamed and embarrassed that a man of God . . . could go over and say he is going to support racism in the name of Christ."

Falwell said he spent his time in South Africa talking to nonwhites from every walk, and that they told him that the government of President Pieter W. Botha is their only hope of reform. He also said Botha had assured him that he is committed to abolishing discrimination.

"Both the president and the foreign minister said that apartheid is wrong and that they were moving to a policy of reform," Falwell said.

"Black leaders said that P.W. Botha is the only one who gave them any ray of hope. They expressed the fear that, if he was pushed too hard, his own government would vote him out, and all the reforms they had enjoyed would be undone," he added.