JOHANNESBURG, JULY 3 -- Fifty members of South Africa's white Afrikaner community plan to fly out of here Monday for a conference with leaders of the underground African National Congress, which is trying to overthrow this country's Afrikaner-led government.

The secretly organized meeting in the Senegalese capital of Dakar will be the first between a group of Afrikaners and the outlawed black nationalist movement.

According to organizers of the conference, the Afrikaner group consists of prominent academics, writers and professional and business people who all question or reject South Africa's apartheid system of strict racial segregation and white minority domination and believe whites and blacks should negotiate a new deal for their strife-torn country.

They will spend four days in Dakar discussing the process of political change and the nature of a postapartheid society with members of the African National Congress' national executive.

The group is scheduled to visit Ghana and Burkina Faso as well as Senegal, and is expected to meet the presidents of all three countries in a two-week tour. For most it will be their first venture into black Africa, which regards South Africa as a pariah state and usually does not admit its white citizens.

Frederik van Zyl Slabbert, former leader of the liberal white opposition in the South African parliament, who is leading the group, said in an interview before leaving ahead of the rest last night that he regarded the meeting as "an historic occasion.

"For the first time the descendants of Afrikaner and African nationalism will be talking about ways of finding a common fatherland and a common government," said Slabbert, an Afrikaner and former apartheid supporter.

The conference and tour have been arranged by Slabbert's new organization, the Institute for a Democratic Alternative for South Africa, which he formed after quitting the racially segregated parliament early last year to pursue interracial conciliation through extraparliamentary politics.

Last week the institute held a first-time meeting between Afrikaner students and young black activists from ghettos around the country.

Next week's conference has been organized jointly with a French civil rights group, the Liberty Foundation, headed by President Francois Mitterrand's wife, Danielle. She is expected to accompany the Afrikaners on their West African tour.

Breyten Breytenbach, an Afrikaner writer who lives in exile in Paris and was imprisoned for seven years in South Africa for attempting to found a revolutionary movement, is on the board of the French foundation and played a key role in establishing contact with the West African leaders. He, too, will accompany the group.

The conference was organized secretly to avoid government action to prevent its taking place, but news of it leaked yesterday and was published in South African newspapers today.

Slabbert then issued a statement confirming that it was to take place. The African National Congress also issued a confirmation from its headquarters in Lusaka, Zambia.

President Pieter W. Botha has denounced whites who have held meetings with the ANC in the past, suggesting that to do so verges on treason by giving credibility to an organization committed to trying to overthrow the state.

In September 1985, a group of white South Africans, most of them of British descent, met with ANC leader Oliver Tambo in Lusaka.

President Botha has shown particular anger at Afrikaners wanting to meet with the outlawed group. Two years ago he ordered that the passports of a group of Afrikaner students from Stellenbosch University be seized as the students were about to leave for Lusaka.

Since then a few individual Afrikaners have held secret meetings with members of the ANC while traveling abroad, including the chairman of the powerful Broederbond secret society, Pieter de Lange. This is the first time since the Stellenbosch students were stopped that a group has tried to do so.

Home Affairs Minister Stoffel Botha said tonight that the government was aware of the group's plans, but refused to say whether it would seize their passports or take other action to block the conference.