News of his sudden death brought hundreds of youths onto the streets of the biggest city, Lagos, setting fire to tyres and throwing stones. Police responded with tear gas, and rioting went on into the early hours of the morning.

"A nation in agony," says BBC Lagos correspondent, Hilary Andersson.

A government statement said Chief Abiola had died of an apparent heart attack at 1600 local time (1500 GMT) during a meeting with Nigerian and United States officials in the capital, Abuja.

BBC Lagos correspondent Hilary Andersson: Unrest on streets

A US state department official confirmed Chief Abiola had just begun discussions with US envoys, when he began coughing heavily and collapsed. A doctor was called in and Chief Abiola was rushed to hospital.

The un-named official said: "The US has no reason to doubt that the sudden death of Chief Abiola was due to natural causes."

The Nigerian authorities have announced that a full post-mortem will be held into how he died.

Pictures released of Chief Abiola meeting the UN Secretary-General, Kofi Annan, last week, showed him in apparent good health, although he had lost weight. He has suffered bouts of illness in jail.

Suspicions mount

The BBC Lagos correspondent says Nigerians are shocked and suspicious, after the sudden death only a month ago of the military leader, General Sani Abacha.

Chief Moshood Abiola was the presumed winner of Nigeria's 1993 elections, which were annulled by Nigeria's military government.

A year later Chief Abiola was arrested and had been in detention ever since. His release had been widely predicted for Wednesday - when official mourning ends for the Nigerian military ruler who jailed him, Sani Abacha.

It is believed that the government was trying to get Chief Abiola to give up his presidential mandate in exchange for his freedom, although as of a few days ago he had not committed himself to this in writing.

Transition to democracy

Before their meeting with Chief Abiola, the American delegation held talks with Nigeria's new leader, General Abdulsalam Abubakar.

A statement from the Nigerian side said General Abubakar had given assurances about the transition to democracy.

Last week, his government promised to release all political prisoners, including Chief Abiola.

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