SOWETO, SOUTH AFRICA, NOV. 10 -- South African black nationalist leader Nelson Mandela today defended plans by the African National Congress for a nationwide campaign to force the collapse of government-backed black township councils and to press for the election of a constituent assembly to write a new constitution.

The ANC's plan for "mass action" to force a halt in the councils' operations and its declared priority on winning an elected constituent assembly suggest that South Africa may be headed for more political turmoil as well as deadlock over the next step in talks between the ANC and the government.

Mandela announced that he and the ANC National Executive Committee will meet within a month with the Kwazulu homeland leader, Chief Mangosuthu Buthelezi, and his Inkatha Freedom Party's Central Committee. Mandela also said he will meet one-on-one with President Frederik W. de Klerk Nov. 27 to discuss increasing tensions in the country.

In addition, Mandela said the ANC's ailing president, Oliver Tambo, will return to South Africa from London Dec. 14 after more than a year of recuperating from a stroke, in time for the organization's first national consultative conference within South Africa.

Addressing reporters at his home here after an 18-day fund-raising tour in Asia and Europe, Mandela said the ANC wanted to "prevent South Africa slipping down into another Lebanon" and was trying to defuse the conflict with Inkatha that led to more than 750 deaths in factional fighting last summer.

Still, he strongly defended the ANC's recent call for mass action to force the resignation of all black township councilors, hundreds of whom already have been driven from office, to seek redress of their political and social grievances.

He said the ANC had rejected two appeals from de Klerk to call off its mass action campaign and told him that "until mechanisms are established to enable the people to express their grievances, the government must accept the fact that there is going to be continued mass action."

Since blacks had no vote and did not participate in government, he said he told de Klerk, "the only alternative we have is mass action in various forms."

The main objective of the latest ANC mass action campaign is to force the collapse of the remaining black township councils, regarded as non-democratic organs of South Africa's apartheid government. Already about one-third of council seats nationwide are vacant and about as many councils are defunct, according to local press reports.

Pro-ANC groups conducted a similar campaign in the mid-1980s that weakened the councils but did not destroy them as an institution. Since the legalization of all anti-apartheid groups last February, scores of town councilors have been killed or burned out of their homes. Others have resigned under strong pressure from township activists seeking to establish "people's power" organs such as civic associations, neighborhood defense units and street committees.

De Klerk has pleaded with the ANC to forgo its mass action campaign and charged this week that it was a form of violent activism that violated agreements between the ANC and the government.

De Klerk also has rejected the idea of holding elections for a constitutional assembly as impractical and unfair since it would place his government in a minority at the very start of the process.

Mandela declined to discuss how much money he had obtained for the ANC during his latest trip abroad, on which he visited six Asian countries, Britain and France.