JOHANNESBURG, DEC. 7 -- The African National Congress has shifted its ground on international sanctions against South Africa, appealing to the European Community to maintain them for another two or three months, then to consider phasing them out if the government meets a number of key ANC demands.

Nelson Mandela, deputy president of the ANC, put this proposal to European Community leaders for consideration at a Rome summit next week in a letter the ANC released here Thursday night.

The ANC's international affairs director, Thabo Mbeki, is reported in South African newspapers to have addressed a closed meeting of diplomats in New York along similar lines sometime this week.

A draft policy document setting out the new sanctions strategy is also said to have been circulated among the ANC's "top and middle leadership" and endorsed by them.

Neither Mandela's letter nor the draft policy document lists the demands the ANC wants linked to the lifting of sanctions. Mandela said the ANC is still discussing these "specific measures" and will convey them to the European leaders later.

The measures are expected to include the repeal of security legislation, tighter control over the police, concessions to ANC demands on the structure of future constitutional negotiations for a new, nonracial system of government, and a role for black political movements in running the country during any transition phase.

Political analysts here say the policy shift stems from a recognition by the ANC that national and international developments have spelled the end of sanctions, and rather than fight futilely to keep them in place, the ANC should take control of their dismantling and turn the process to its advantage.

By linking their removal to a demand for further specific changes by President Frederik W. de Klerk, the ANC can seize the initiative once more and step up pressure on the government to concede important pre-negotiation points, according to this view.

Key ANC leaders fear that at the moment the government has the upper hand. De Klerk has won international acclaim for his reforms, easing world pressure on South Africa, and the ANC suspects he is stalling on talks while ongoing violence in the townships sullies the ANC's image and causes growing disillusionment in the black community.

According to the analysts, the ANC has decided on a two-pronged strategy to revive pressure on de Klerk -- a campaign of mass street action to dramatize the ANC's demands that began with peaceful protest marches through three cities Thursday, and the linking of an end to sanctions to a list of demands.

Both appear timed to press de Klerk into making another major reform announcement at the opening of the 1991 session of Parliament on Feb. 1 -- just as he did with his dramatic speech legalizing the ANC when he opened the 1990 session last Feb. 2.

In his letter to the European leaders, Mandela asked them to postpone any decision on sanctions until February or March.

"We are discussing the question of sanctions with the aim of ensuring that the international community continues to exert pressure for faster movement forward towards the end of the apartheid system," Mandela wrote.

"We are discussing specific measures in this regard and have begun consultations with other democratic formations inside our country . . . to evolve a position common to the majority of the anti-apartheid forces in the country.

"We would be honored to share with you our detailed views on these matters at a later stage, but soon, so that, if possible, we avoid adopting contrary positions which could undermine the consensus that obtains concerning what needs to be done to effect the necessary changes in South Africa."