Nobel laureate Desmond M. Tutu, sounding a theme of racial conciliation and reiterating his commitment to peaceful change, offered today to act as a mediator between South Africa's white-minority government and the country's main black underground movement, the African National Congress.

Speaking at his enthronement as the first black Anglican bishop of Johannesburg, Tutu also said that he favored increased foreign investment in South Africa under stringent conditions but that if the country's apartheid system of racial segregation is not being "actively dismantled" within 18 months to two years he will join the growing call for divestiture.

A number of South Africa's leading white businessmen were in the racially mixed congregation of 1,500 that packed the cathedral of St. Mary the Virgin in downtown Johannesburg for the candlelight, 3 1/2-hour ceremony, blending old English pageantry and African hymns. The service presented Tutu with the most important white audience he has addressed in his own country, where most whites regard him as a troublemaking extremist.

He seized the opportunity to try to improve his image among them, saying that he is as concerned for his white parishioners as for his black ones and that as pastor to both wants to play the role of conciliator.

"I love you all deeply -- give me a chance to show you that I do," Tutu told the white church members, some of whom had tried to block his appointment as their bishop. He added that when they knew him better they might find he is "a slightly more lovable ogre" than they realized.

Tutu said he will work for more multiracialism in the parishes of his 100,000-strong diocese, his church's largest, encouraging white clergymen to learn African languages and stationing some in the black ghettos to dispel "erroneous notions" about whites that people there have gained from their encounters with the authorities who enforce apartheid.

It was difficult to judge the white reaction in the formal atmosphere of the religious ceremony. Tutu has some enthusiastic white supporters who were disproportionately represented in the congregation, and they were visibly delighted. Others listened impassively to their new bishop's hour-long message of Christian love mixed with political admonitions, parts of which were made inaudible by a poor amplifying system.

Tutu's offer to mediate between the government and the African National Congress comes as the first indirect contacts are being made between them since the congress was outlawed 25 years ago.

Several secret meetings between influential government supporters and exiled members of the congress have been held in Lusaka, Zambia, during the past two months, and more meetings are scheduled soon.

Although no common ground appears to have been established at these meetings, and President Pieter W. Botha has forbidden any state officials or members of Parliament of his ruling National Party to attend further meetings, the contacts have led to speculation that a deal may be in the making. Few informed observers expect this, but Tutu, who has close contacts with the exiled leaders, made it clear today that he welcomes the contacts.

"Let us talk together, black and white," Tutu said. "Thanks be to God that the government may be doing so with the ANC. I offered myself long ago as a go-between. I renew that offer in all seriousness, for we will not have security and peace until we have justice, and we will not have that without the participation of the premier black liberation group, the ANC."

Tutu's appeals while abroad for political, diplomatic and economic pressure on South Africa to force it to dismantle apartheid have been the main reasons for white animosity. The depth of the hostility was revealed when senior white clergymen in the Anglican Church, traditionally one of South Africa's most liberal, tried to block his election as bishop of Johannesburg.

Dealing with the sanctions issue, Tutu said today that he would not call for foreign divestiture if his conditions are met within 18 months to two years.

"I give notice that if by then apartheid is not being actively dismantled, then for the first time I will myself call for punitive economic sanctions whatever the legal consequences may be for doing so."

Tutu's conditions are that black workers should not be migratory laborers but should be housed with their families near their place of employment; they should have free labor union rights; regulations restricting the movement of blacks into cities should be lifted, and there should be massive investment in black education and training. Forced population removals should be stopped, and the government should end its policy of stripping blacks of their South African citizenship and making them nationals of tribal "homelands."

Referring to laws that split African families, Tutu said, "It is that we, too, are just ordinary human beings. We, too, love to be with our wives every day. We, too, want our children to rush out to meet us as we come back from work . . . .

"These are not extravagant demands. They are the expectations of any human being. We want to have a new kind of South Africa where all, black and white, can walk tall together into the glorious future which God is opening up before us."