Burundi's army imposed a state of emergency throughout the country and ordered a dusk-to-dawn curfew. The coup appeared to snuff out the Central African state's experiment with democracy just three months after Mr Ndadaye took office after winning Burundi's first multi-party elections.
'We have just heard that the new President, Melchior Ndadaye, may have been killed together with the speaker of parliament,' Rwanda radio said in repeated broadcasts. It said that hardliners in the 5,000- strong army dominated by the minority Tutsi tribe carried out the coup.
Mr Ndadaye was the first member of the majority Hutu tribe, which comprises about 85 per cent of the population, to become president. His election victory ended decades of military rule.
The radio said Mr Ndadaye may have been executed with parliament's speaker, Pontien Karibwami, the vice-speaker, Giles Bimazubiute, and the Vice-Prime Minister, Bernard Ciza, but this could not be confirmed independently. Belgian radio earlier said Mr Ndadaye and several ministers had been executed. 'Not just Ndadaye, but also the prime minister of Burundi has been executed and a few ministers,' it said.
The Belgian Foreign Ministry in Brussels could not confirm any killings but the coup raised fears of ethnic slaughter in Burundi between the Tutsi and the Hutu tribes.
Officials in Rwanda said the uprising was led by Colonel Jean Bikomagu, the army chief of staff, and the former president, Jean Baptiste Bagaza. But officials monitoring events from the Ugandan capital, Kampala, said it was probably the work of junior officers.
They said Francois Ngeze, a Hutu and former minister, had offered to be president.