Third World leaders today demanded a mandatory economic blockade of South Africa while European Community countries debated a series of limited measures aimed at expressing disapproval of Pretoria's state of emergency.

The calls for mandatory economic sanctions came at a special United Nations conference here timed to coincide with the 10th anniversary of the Soweto riots. Several speakers, including former U.S. presidential candidate Jesse Jackson, criticized the United States, Britain, and West Germany for refusing to take part in the meeting.

In Luxembourg, West European foreign ministers postponed a decision on a limited embargo on the import of South African agricultural goods. They also issued a joint diplomatic protest to South Africa in the name of the 12-nation European Community over this weekend's police crackdown by the white-led government.

The sudden bout of diplomatic activity reflected the deep divisions within the world community over how to bring pressure to bear on South Africa to end the system of racial segregation known as apartheid. Many western countries have resisted growing pressure from Third World nations for mandatory sanctions on the grounds that they would only hurt the black population without assuring political changes.

The chairman of the Organization of African Unity, Senegalese President Abdou Diof, called on the U.N. meeting here to organize "full-scale sanctions" against South Africa in order to force change. He said an oil embargo would severely damage the South African economy, which is dependent on fuel supplied by international oil companies.

A similar call for urgent action came from Commonwealth Secretary General Shridath Ramphal, speaking on behalf of the 49-nation community. A six-month Commonwealth effort to help negotiate a peaceful solution to South Africa's racial conflict ended last week with a report concluding that international economic pressure offered the last opportunity to avert a "racial conflagration."

The U.N. conference, which ends on Friday, is unlikely to have much effect on the western governments that have made clear in advance they will not be bound by its recommendations.

In the absence of any official U.S. delegation, Jackson won loud applause from the representatives of more than 100 countries by criticizing western nations for allegedly propping up the apartheid system. The former candidate for the Democratic nomination for president headed an unofficial delegation that included American entertainer Harry Belafonte.

Today's meeting of West European foreign ministers in Luxembourg marked the first time the European Community has debated possible economic sanctions against South Africa since last September. On that occasion, members of the trading bloc took a series of largely symbolic measures, including the withdrawal of their military attaches from Pretoria and a ban on the sale of arms.

European Community embassies in South Africa were closed for one day today as a gesture of sympathy with the black majority over the deaths at Soweto. A joint diplomatic protest was also delivered to the South African ambassador in The Hague by the Dutch government, which is acting president of the community.

The limited list of measures against South Africa being considered by the community will be presented to a meeting of heads of government in The Hague on June 26 and 27. Measures include a ban on the importation of fruit, vegetables and wine from South Africa. The strongest support for firm action has come from Denmark and Ireland and the strongest opposition from Britain and West Germany.