JOHANNESBURG, FEB. 11 -- Gunmen opened fire with automatic rifles Sunday on two buses carrying supporters of Chief Mangosuthu Buthelezi from a prayer meeting in eastern Natal province, killing 17 people, and black factional leaders sought today to keep recent peace efforts alive.

Local officials of Buthelezi's Inkatha Freedom Party blamed elements affiliated with the African National Congress for what they called a carefully staged ambush, which also left 29 injured.

Police rushed in reinforcements and pleaded for restraint by thousands of angry Inkatha supporters who gathered this morning at the site. The incident was the bloodiest between backers of the ANC and Inkatha since Buthelezi and ANC leader Nelson Mandela met Jan. 29 at a "peace summit" and called for an end to the conflict. The post-meeting death toll now is almost 40.

Fighting in Natal is estimated to have taken between 3,000 to 4,000 lives in five years. It spread last summer to townships around Johannesburg, where almost 1,000 have died.

Mandela and Inkatha Party Chairman Frank Mdlalose issued a joint statement today expressing their "shock and horror" and pledging "all our efforts" to discover the causes of the clashes.

It was not immediately clear if the joint efforts would succeed in stopping Inkatha retaliation. The incident highlighted the limited influence national leaders are able to exert on their followers in areas where memories of grievances run deep.

The latest incident took place in a remote pro-Inkatha district known as Taylor's Halt, outside Pietermaritzburg in central Natal province. Buthelezi suggested last March that he and Mandela meet there but the local ANC vetoed it.

The local Inkatha leader, David Ntombela, said the victims were part of a group of 90 Inkatha members who were returning from a prayer meeting. He accused "ANC people" of mounting what he desribed as "a well-planned ambush" in which automatic rifles as well as other rifles and pistols had been used.

"How long must we accept these attacks on people who have come to a prayer meeting? Enough is enough," Ntombela told reporters. Another Inkatha leader, Velaphi Ndlovu, said the ambush occurred at the same place as one in 1987 in which 13 children died and touched off a major escalation of the fighting.