Zulu Chief Gatsha Buthelezi, addressing the only political rally the authorities have allowed in Soweto since imposing a state of emergency 17 days ago, today denounced the U.S. House of Representatives' vote favoring economic sanctions and hinted he might eventually join the government's proposed multiracial advisory council.

After the rally, which was ringed by heavily armed police, soldiers and Buthelezi's private Zulu army, violence broke out between his supporters and those of the rival United Democratic Front. At least two persons were shot by Buthelezi's followers, witnesses said, and a bus was fire-bombed.

The state Bureau for Information, the only authorized source of news during the emergency, reported that four blacks died in overnight violence, bringing to 85 the death toll from civil unrest since the nationwide security crackdown began June 12.

Buthelezi, who considers himself a political moderate, advocates economic growth, free enterprise and nonviolence, although members of Inkatha, the Zulu political and cultural movement he heads, have been involved in increasingly violent clashes in recent months with the UDF, the militant antiapartheid coalition on Inkatha's left. At least one Inkatha supporter died in the clashes here last week.

Buthelezi's rally, which drew about 15,000 to Jabulani Stadium, was billed as a "prayer meeting" in line with emergency restrictions on political gatherings. But the meeting, which took place with the approval and protection of the police, had the trappings of a carefully staged political event.

About 40 busloads of supporters were brought in from KwaZulu, the black Zulu "homeland" 300 miles away that Buthelezi governs as chief minister. There were political signs, including one that read: "American House of Representatives, Rethink! Rethink! Sanctions Will Kill Black People."

Hundreds of Zulu "warriors," waving clubs, spears and shields, danced and marched around the stadium until the chief dramatically descended in a white helicopter and emerged, preceded by armed white security guards. Police made no attempt to stop reporters from covering the rally, as they have for all similar meetings since the emergency was declared.

The white-ruled government looks to Buthelezi as a key figure it hopes to win over. It sees him as the most viable black opposition to the power of the UDF and the outlawed African National Congress.

Pretoria would also like Buthelezi to endorse and join the multiracial National Statutory Council, to be chaired by President Pieter W. Botha, which officials contend will begin to negotiate South Africa's political future. But Buthelezi, who has carefully avoided the government's embrace, made clear today that it would have to lift the emergency, release imprisoned black nationalist Nelson Mandela and scrap the constitution, which excludes blacks from parliamentary participation, before he would join the council.

"I am nowhere near saying yes or no to the national council," Buthelezi said. "We do not have to rush into a decision now."

But, he added, "if it is possible for people like my brother, Dr. Mandela . . . to choose whether or not they want to enter the council, and if the council indeed is about negotiating a new constitution acceptable to all South Africans, and if the council is given bite and is more than a talking shop, then it would be foolhardy for blacks to reject it out of hand."

He said the House vote two weeks ago in favor of requiring all U.S. companies to pull out of South Africa in six months unless certain conditions are met was "tragic." Black people, he said, "want more jobs, not less jobs. They want more investment, not less investment."

"There are many in the United States and in other foreign government circles who simply do not want to hear the voices of the black masses in South Africa," the chief added. "I am quite sure that winning a debate in the American House of Representatives is far more important to some United States politicians than your future."

Buthelezi also denounced "black-on-black" violence, which he said dilutes black power and harms efforts to end apartheid peacefully. "If there was black unity across the board, we would . . . bring about the total liberation of our country through the politics of negotiation," he said.

While the rally came off quietly, busloads of Inkatha followers leaving Soweto clashed with young UDF "comrades" in the Orlando West section, a UDF stronghold. Shots were fired from one bus at a UDF crowd, at least two of whom fell wounded, witnesses said.

The crowd stoned a second bus, which veered into a gas station where a melee ensued that was broken up by riot police.

The Bureau of Information reported that two buses rammed into the bus that had been fire-bombed, injuring 34 people.

At St. Paul's Anglican Church not far from the Buthelezi rally, the Rev. Paul Moore, Episcopal bishop of New York, preached a sermon in which he said all blacks he had met here had told him they were prepared to suffer the consequences of economic sanctions to bring down the apartheid system peacefully and avert a civil war.

Moore said many Americans supported that position and were "deeply ashamed" of President Reagan, who has opposed sanctions.