Three bombs exploded in supermarkets in the port city of Durban tonight, causing no casualties but conveying a warning that South Africa's year-long racial unrest may take on a new dimension of urban terrorism.

At the same time the country's normally apolitical business community showed an increasing inclination toward becoming a pressure group for reform as the chief executives of 91 major corporations issued a joint statement calling on the government to end race discrimination and begin negotiations with black leaders about political power sharing.

The Durban bombings were the first such attacks in South Africa in three months, and the first ever in shopping complexes.

There was no claim of responsibility for the explosions but they were widely assumed to be the work of the outlawed African National Congress, which has carried out previous bomb attacks.

A meeting of the exiled nationalist group decided in June to cease limiting guerrilla targets to government installations and security personnel, and to be less concerned about trying to avoid civilian casualties.

Tonight's supermarket attacks, if perpetrated by the ANC, still revealed a measure of caution, with all the bombs exploding after closing time when the stores were empty, but some observers saw them as a warning of a new phase in the group's insurgency campaign.

The three explosions occurred at intervals of 15 to 40 minutes in South Africa's third largest city, sending large forces of police and ambulances rushing from one incident to the other for more than two hours.

A fourth bomb was found in another supermarket and defused before it exploded.

Managers of the supermarkets said damage was slight.

There was another indication of a new readiness to attack white civilians when a gasoline bomb was hurled into a grocery store in a white suburb of the eastern Cape Province city of Uitenhage today. An Asian woman shopper was injured. Police reported that black youths also gasoline-bombed an automobile in a white suburb of Uitenhage, injuring a white man.

The chief executives who issued today's joint statement represent three-fourths of South Africa's major corporations. The statement will appear in full-page advertisements in all the country's leading newspapers over the weekend.

The document commits the businessmen to verbal support for reform but no action.

The statement calls for abolition of statutory race discrimination, negotiations with black leaders about reform, the granting of South African citizenship to blacks and the restoration of the rule of law.

The executives of 19 top corporations refused to sign the statement. Most of the leading Afrikaner companies were conspicuously absent.

Anthony H. Bloom, one of the signatories, admitted it was "not earth-shattering," but said he felt it was an important first statement of consensus by the majority of the country's top businessmen against the government's apartheid system of segregation.

Bloom said he believed threatened economic sanctions, the divestment campaign and the financial crisis that hit South Africa after American bankers refused to roll over the country's short-term loans last month had aroused new concern among businessmen.

The announcement of the joint declaration follows a statement by the president of the Association of Chambers of Commerce, Raymond Parsons, yesterday that key businessmen are engaged in negotiations aimed at setting up talks on reform between black and white leaders.

In Johannesburg, Nobel laureate Bishop Desmond Tutu began a 24-hour hunger strike to protest the use of soldiers to quell unrest in black townships. He joined 21-year-old university student Harald Winkler who was in the the 11th day of a fast against the government's conscription policies.

There were reports of sporadic unrest from many parts of the country today, and police said 99 more people were arrested under the emergency regulations this week. More than 3,000 of those arrested since the emergency was proclaimed in 36 towns and cities last June have been released, leaving 574 still detained.

The Minister of Law and Order, Louis le Grange, issued a new proclamation today banning open-air meetings countrywide until next March. Only sports events may be held outdoors.