News of the sudden death of the imprisoned opposition leader, Chief Moshood Abiola, has shaken Nigeria.

A number of people are reported to have been killed in clashes with police as angry protests continue in southern Nigeria, the political stronghold of Chief Abiola.

At least two people were killed in overnight riots in Lagos. Hundreds of people had poured onto the streets on Tuesday night after hearing news of the opposition leader's death, burning tyres and throwing stones. Police responded with tear gas and fired into the air to disperse crowds.

BBC Lagos correspondent, Hilary Andersson: A nation in agony

Lagos and the rest of ethnic Yoruba region in the south-west form the political stronghold of Chief Abiola, while the Hausa-speaking north of Nigeria has been the home of most of the country's rulers since independence, including General Abdulsalam Abubakar.

Between five and seven people are estimated to have been killed in clashes on Wednesday morning in Abiola's hometown, Abeokuta.

Chief Abiola, who was due to be released from prison after a four years in solitary confinement, died of an apparent heart attack after being taken ill during talks with a visiting United States envoy on Tuesday.

He is expected to be buried in Lagos later on Wednesday, following a post-mortem. Nigeria's ruling military council is also due to meet to discuss the aftermath of his death and plans for a transition to civilian rule.

Mystery surrounds death

Many of Chief Abiola's allies have voiced suspicions over the manner of his death, and his daughter Wuru said she believed he was killed.

Hilary Andersson: Unrest on streets

"(He died) either because medical neglect brought on a heart attack or because they poisoned him," she said.

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.

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.

But President Clinton dismissed speculation that the death was a result of foul play, and the United Nations Secretary-General, Kofi Annan, called for calm.

Attention is now focusing on the new military leader, General Abubakar, who is scheduled to make a speech to mark the end of official mourning for his predecessor.

Government statement announces death

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

Kofi Annan: Elections must be credible

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.

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.

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