The African National Congress and a rival opposition group, the Pan Africanist Congress, should work together to force the end of apartheid and a new constitution, a PAC leader said Saturday.

A de facto alliance by the two groups would reverse decades of rivalry within the black movement opposed to apartheid, South Africa's system of racial segregation and white dominance.Clarence Makwetu, recently named acting head of the militant PAC after the death of its president, Zeph Mothopeng, made the statement a day after a similar call by ANC leader Nelson Mandela.

Mandela is scheduled to meet Tuesday with President F.W. de Klerk to discuss planned negotiations on ending apartheid and giving political power to the black majority.

The meeting comes amid building tension between the ANC, the largest opposition group, and the white-led government. The two sides have held peace talks since May aimed at removing obstacles to the constitutional negotiations, planned for next year.

But the government recently has condemned the ANC's campaign of protests and strikes against apartheid as a form of violent rebellion.

Mandela has called for increased protests to pressure the government to continue its reforms. He said South African blacks, prohibited by law from voting in national elections, must use mass action to express their grievances.

On Friday, Mandela said he would demand election of a special assembly to write a new constitution and formation of an interim government to help ease the transition to a non-racial political system.

The government has rejected similar calls by other anti-apartheid leaders, saying it must remain intact to maintain stability while negotiations take place.

Mandela's demands indicate a hardening of the ANC's negotiating position. He also questioned de Klerk's willingness to proceed with negotiations, and called for black opposition groups such as the ANC and PAC to band together in seeking change.

He said the PAC's choice of Makwetu as acting leader gave him hope for unity. The two men spent 10 years together as prisoners on Robben Island.

Makwetu, speaking at an African trade summit in Swaziland, said Saturday he agreed with Mandela that their organizations should work together to oppose the government.

He said he would work to unite various opposition groups so that "we can confront de Klerk with a demand for a constituent assembly."

De Klerk has pledged to end apartheid and negotiate a new constitution, but also has called for protection of minority rights.