DEMONSTRATORS burned buses and set up barricades in the Zairean capital, Kinshasa, yesterday in protests against the appointment of a new prime minister by President Mobutu Sese Seko.One person was seriously hurt when a man driving at high speed opened fire on a group of demonstrators from his car in the populous Kintambo section of Kinshasa, witnesses said. Angry crowds denounced the president's latest maneuver in a continuing power struggle with the increasingly outspoken and powerful opposition in this vast central African country. Units of the pro-Mobutu civil guard and elite presidential forces patrolled the city in armored personnel carriers, attempting to break up anti-Mobutu protests. The opposition Sacred Union coalition was due to meet later today to formulate a response to President Mobutu's naming of Mungul Diaka as Zaire's fourth prime minister this year. Mr. Mungul replaces Etienne Tshisekedi, Mobutu's most outspoken opponent, as prime minister of the country of 40 million people which has been teetering on the brink of chaos since riots last month devastated Kinshasa and several provincial cities, leaving 250 people dead. Western diplomats and Zairean observers said they had feared the worst after Mobutu refused to reappoint Mr. Tshisekedi. "I fear this will lead to real confrontation. None of the real opposition parties will accept this," one Western diplomat commented. "Mungul Diaka is very charming but he is not front-rank. He has no popular support in Kinshasa." For 30 years Mungul has veered sharply from one political stand to another, twice falling out with Mobutu and twice returning to the fold. There were fresh outbursts of rioting in the southern copper-belt Monday night, prompting former colonial power Belgium to evacuate some 300 of the foreigners remaining the region's main city of Lubumbashi.