The sudden death in custody of Nigeria's leading opposition politician, Chief Mashood Abiola, just before his widely anticipated release, could not have come at a worse time for the military dictatorship which had imprisoned him.

The regime was in the throes of trying to convince a sceptical world that it was about to democratise itself, in the hope of getting US and European sanctions on Africa's most populous country lifted. Only days ago the UN Secretary General, Mr Kofi Annan, was told that it would release all political prisoners, and 30 were already free.

Now Mr Abiola is dead, apparently of a heart attack, on the very day he was visited in jail by a senior US delegation. The US envoys had earlier held "very productive" talks with Nigeria's new military leader, Gen Abdulsalam Abubakar.

Mr Abiola was the only Nigerian leader who could claim a popular mandate to lead the country, since he was clearly the front runner in the 1993 presidential election. The army cancelled that result when the military saw their preferred candidate losing to Mr Abiola. He then fled the country, insisting that he did not want his presence to provoke a blood-bath like the 1967-70 civil war.

The following year he returned, only to be jailed for "treason" by his former friend, Gen Sani Abacha. Ironically, it was Gen Abacha's own death just four weeks ago, also by a heart attack, that provided the opening for the tentative democratic reforms offered by Gen Abubakar.

While the authorities have insisted that his death was due to natural causes, and the US visitors have accepted that "foul play was not involved", some of Mr Abiola's extensive and divided family have suggested he was murdered.

Mr Abiola's daughter Wura said the family believed his death was "very suspicious". This view will probably be echoed by many of his supporters, particularly since his most politically active wife, Kudirat Abiola, was mysteriously assassinated on a Lagos street in 1996.

The turbulent life of Mr Abiola, a tycoon turned politician, began on August 24th 1937, at Abeokuta, 40 miles north of Lagos. He was born poor, but won a scholarship to the University of Glasgow and took a degree in economics.

A Muslim member of the Yoruba ethnic group, he seems to have married, or at least fathered children, by many more women than the four permitted by his religion. He used to joke that his first step up in the world was to marry his relatively wealthy first wife, Simbiat, who gave him the first egg he ever ate.

Mr Abiola's personal life kept him in the public eye before he went into politics. In 1992 a New Jersey judge ordered him to pay £13,000 a month in child support to a woman who said she was his wife. Mr Abiola argued that the woman was just one of his 19 concubines.

Divisions among family sharpened while he was in prison, with some of them backing his continued claim to the presidency, while others simply wanted to see him free and back in business.

His first major business break had come with the telecommunications multinational, IT&T, where he was a vice president and chairman for Africa and the Middle East from 1971 to 1988. Some of his critics - and not just the military - blame the parlous state of the Nigerian telephone system on his IT&T tenure, claiming he used second-rate materials and siphoned off the surplus cash.

However he made his money, he was soon investing it successfully in publishing, shipping and fishing, and became a dollar millionaire.

He was a stocky man with a cheerful, flamboyant demeanour. Before the 1993 elections, he had become a public figure through charitable activities, serving on Nigeria's Olympic Committee, and running a campaign calling for western countries to compensate Africa for the slave trade.

He won the Social Democratic Party presidential nomination in 1993, with a wildly populist promise to end poverty within six months.

But if his significance for Nigerian democracy has been exaggerated, it was symbolically magnified by his five years in prison, and will be sanctified by his death.