Nobel laureate Desmond M. Tutu, in his first public statement since returning home from a widely publicized three-month international tour, today called for a campaign of "persuasive pressure" requiring foreign companies to attach reformist conditions to their investments here for a test period of 18 months to two years.

Tutu, addressing a press conference in Johannesburg, said he was not yet campaigning for the withdrawal of foreign capital from South Africa but said a campaign of political, diplomatic and economic pressure against South Africa is "our last chance to avert a bloodbath."

If the conditions were not met within the specified time, "the pressure must become punitive and economic sanctions should be applied," Tutu said.

It was the first time the Nobel laureate has adopted a specific position on the divestiture issue that affects a large number of American companies operating here.

His call for "persuasive pressure" stands in marked contrast to the Reagan administration's policy of "constructive engagement," which is based upon a premise of close cooperation with the South African government.

As a South African it is illegal for Tutu to advocate economic sanctions against his country. And, until now, he has been deliberately vague on divestiture while giving a general impression that he favors increased foreign pressure to force the white-minority government to dismantle its segregationist system called apartheid.

Even the call which Tutu made today, with its implicit threat of sanctions, could mean that he is running a risk of prosecution, but he apparently has judged that his increased international status since being awarded the Nobel Peace Prize has afforded him a degree of protection.

In calling for conditional investment, Tutu has opted for a middle course between divestiture and appealing to American companies operating in South Africa to adhere to a code of conduct called the Sullivan Principles.

This code, to which about 120 of the 350 American companies operating in South Africa subscribe but which Tutu considers inadequate, was devised by the Rev. Leon L. Sullivan of Philadelphia.

It requires companies do such things as improve working conditions for black employes and ensure that they are paid the same wages as whites.

Last month, a meeting of subscribing companies agreed to lobby for social changes as well.

Tutu wants a more overtly political and time-related kind of pressure.

Under his "persuasive pressure" campaign, he said, investing companies should demand that specific reforms be made within a certain time.

The changes should include the abolition of the migrant labor system and the housing of black workers with their families, ending the pass laws which prevent blacks from moving freely into the cities, unrestricted labor union rights for all and investment in black education training.

"If these reforms are not implemented within the time limit, then the pressure must become punitive and economic sanctions should be imposed," Tutu said.

He said his proposal was intended to "show that we are trying to be reasonable. We are saying, 'Please can you give us a way of changing apartheid reasonably peacefully.' "

But the Nobel laureate warned that he might reassess his standpoint on divestiture in less than two years, "because I think that we are having a crisis in this country that is deepening."

Tutu, who met with three presidents, including President Reagan, two monarchs and an array of prime ministers and foreign ministers during his sojourn abroad to receive the Nobel prize, returns as the most widely acclaimed black man in his country's history.

But he firmly renounced any claim to political leadership of his people.

He said his major concern now would be to serve as the new Anglican bishop of Johannesburg, a post he assumes next month.

"I am a political leader by default," Tutu said, "because the real leaders of our people are either in prison or in exile."

He added that as bishop of South Africa's largest and most racially mixed city, "I want to be pastor to all the people and to care for all of them. I am concerned for both black and white."

He said he hoped his diocese would become "an effective instrument for campaigning for justice and reconciliation."

Tutu, whose calls for increased pressure on South Africa have made him a hated figure to many whites, fielded a number of hostile questions with a mixture of bantering humor and reflective sadness.

"I love to be loved perhaps more than most," he said, "and it is a great ache that people deliberately misconstrue what I say. Perhaps one day they will discover that I am not the ogre they think I am."

A moment later the mercurial bishop responded with peals of laughter to a reporter's suggestion that President Pieter W. Botha might also win the Nobel Peace Prize, as Menachem Begin did. He then added with a sudden two-edged soberness: "They [the ruling white Afrikaner minority] are God's children as well, and, belonging to the Church of God, Botha is my brother whether he likes it or not, and I have a responsibility for him."

In his prepared statement, Tutu emphasized that the growing anti-apartheid sentiment in the world should not be equated with racial feelings against whites.