Leaders of the European Community today called on South Africa to free Nelson Mandela and other imprisoned black political leaders "unconditionally," and to lift the ban on Mandela's African National Congress and other proscribed black political parties.

The leaders ended their latest summit by sidestepping the controversial issue of imposing economic sanctions against Pretoria.

According to diplomats, sanctions are backed by nine of the community's 12 governments, but opposed by Britain, West Germany and Portugal.

South Africa rejected any threat of sanctions by the community, and hinted that it might take retaliatory economic action against neighboring black-ruled nations, Reuter reported from Johannesburg.

A watered-down compromise agreed to after two days of meetings here proposed only to consider imposing sanctions three months from now if the South African government had not shown willingness to enter into negotiations with its country's black majority.

The leaders said the freeing of Mandela and other black political leaders and the legalizing of their parties are necessary conditions to begin the dialogue and negotiations which they deem the only hope for South Africa's avoiding a civil war.

To convince the South African government of the need to begin talks, the community leaders also empowered British Foreign Secretary Sir Geoffrey Howe, who next week takes over as chairman of the community's Council of Ministers, to visit southern Africa "in a further effort to establish conditions in which the necessary dialogue can commence."

Howe's chances of success are dim, however, according to diplomats here. Not only is he expected to get little cooperation from the South Africans, but black activists also have made it plain that they are not interested in talking with any more empty-handed emissaries from abroad.

John Makatini, international director of the ANC, today termed Howe's mission a "nonstarter" and said that Mandela, who has been in prison for decades, would be told not to meet Howe. "No authentic black spokesman will meet with him," Makatini said.

During the three months allotted for the mission, the European Community also plans to consult with "other industrialized nations," specifically the United States and Japan, on "further measures" which "might be needed" if South Africa does not respond.

These "further measures," the council agreed, would involve a ban on new investments in South Africa and an embargo on imports of coal, steel, iron and gold coins from the South African nation.

Under what conditions these "further measures" might be invoked, however, remained very much the subject of disagreement tonight among the government heads as they departed.

Dutch Prime Minister Ruud Lubbers spoke of the measures as economic sanctions that he said all participants had agreed "around the table" would be imposed if South Africa rejects the community's conditions to begin to end apartheid.

"We do have a concrete package of sanctions ," he said, "and we are past the stage where one or two members would rule out economic sanctions. We have agreed on this."

That interpretation, however, was rejected tonight by both British Prime Minister Margaret Thatcher and West German Chancellor Helmut Kohl.

"No sanctions have been decided," Kohl said. "There is no ultimatum."

Thatcher used almost identical terms, insisting that "There is no ultimatum, no automatic implementation of the measures.