African radicals and their allies today pushed for a total economic boycott and ban on nuclear cooperation against South Africa despite the reluctance of the major Western nations.

Representatives of the United States, Britain, France and West Germany - all of whom have large investments in South Africa - argued at the United Nations-sponsored world conference for action against apartheid here that economic sanctions would be self-defeating now that South Africa has shown some flexibility about the eventual independence of Southwest Africa (Namibia).

South African liberation movements and their Third World and Communist backers, countered that the only language South Africa's white-minority regime understands is tough action from the outside world and armed struggle by the blacks inside the country.

The major Western powers are particulary reluctant to give in to demands that South Africa be subjected to mandatory economic sanctions and a ban on all forms of nuclear cooperation, peaceful as well as military.

The radicals are seeking the same kind of total economic sanctions on South Africa as have been applied to Rhodesia since the former colony unilaterlly declared itself independent of Britain in 1965.

Application of such a total trade boycott would hinder Western investment in South Africa.

France and West Germany are known to have important contracts to provide South Africa with nuclear power plants for peaceful purposes, so they do not favor any blanket ban on nuclear cooperation.

Monday, however, French Foreign Minister Louis de Guiringaud confirmed earlier Soviet charges that South Africa was about to explode a nuclear device and warned that any such move would harm French-South African relations.

A message from President Carter sought to reconcil the moderation of the West with demands for change in South Africa's racial policies.

Carter reiterated his earlier warning that South Africa must "embark immediately on the progresstive transformation of its society to one that guarantees full and quitable participation in the political process to all its people.

"If there continues to be no significant movement ot begin such fundamental change," his message said,"then this will operate to the detriment of the constructive relationship we have sought to maintain with South Africa."

The U.S. delegation to the conference was headed by Andrew Young, American ambassador to the United Nations.

Complicating the West's position was the outspoken backing for tough measures against South Africa - some as sweeping as thoss proposed by the radicals - from such countries as Denmark, Ireland and the Netherlands.

Despite these seemingly irreconcilable differences, observers did not rule out a possible compromise by the time the final resolutions are due for adoption Friday.

In a somewhat similar conference devoted to Rhodesia and Namibia, held in Mozambique in May, the Western powers were allowed to express their reservations about individual resolutions. This avoided a vote and allowed the so-called program of action to be adopted by consensus.

News services reported the following other developments :

Kenya's Daily Nation newspaper charged that Nigeria barred a great number of foreign correspondents from attending the Lagos conference.

Rhodesian Prime Minister Ian Smith will meet South African Prime Minister John Vorster in Pretoria Saturday, South African officials announced. The announcement, coinciding with plans for a summit meeting in Zambia of the five "front-line" African presidents - Angola, Botswana, Mozambique, Tanzania and Zambia - raised speculation about new diplomatic efforts to reach a peaceful settlement in war-torn Rhodesia.

In Cape Town, South Africa's ruling National Party approved a plan to share some power with Indians and those of mixed race, while continuing to deny any such role to blacks.