Word of the President's death in what some sources described as an ambush came as troops and tanks reportedly swarmed on the streets of Niamey, the capital of the West African nation. It was unclear whether forces loyal to the Prime Minister, Ibraim Assane Mayaki, had been involved in the death or whether his camp had quelled an army coup.
Mr Mayaki announced in a radio address that the National Assembly had been dissolved and that all political activity had been suspended. He added that a government of national unity would be formed in a few days.
Mr Mayaki said Mr Mainassara had been killed in a "tragic accident", but United States officials speaking in Washington insisted that the the latter had been assassinated by members of his presidential guard.
Nigerien diplomats based in the neighbouring country of Burkina Faso said mutineers had opened fire on Mr Mainassara while he was at Niamey's airport trying to flee the country.
Kofi Annan, the United Nations secretary-general, learnt in Geneva "with dismay" about the killing, a UN spokesman said. "The secretary-general wishes to reiterate his condemnation of such terrorist acts and any attempt to change the government of any country by force or by other unconstitutional means," the spokes-man said. He added that Mr Annan had called on Niger's leaders to work for "an early return to peace, stability and constitutional order".
Mr Mainassara, a former army colonel, himself seized power in a coup three years ago by ousting the country's first democratically elected government. He placed Mahamane Ousmane, president at the time, under house arrest but released him three months later. He said he staged the coup because Mr Ousmane had failed to address Niger's burgeoning economic and political problems.
Mr Mainassara's death comes amid opposition calls for his resignation after the Supreme Court annulled regional elections held in the week and ordered fresh polls. There had been unconfirmed reports of attacks on vote counters. Opposition politicians claimed they were winning the elections and accused the government of inciting the violence.
Mr Mainassara's government quelled several army uprisings and mutinies last year. Two leading opposition politicians, including the former prime minister, and five army commandos were arrested in early 1998 for allegedly plotting to kill the President. Critics say the allegations were fabricated to justify a crackdown on political opponents. (AP)