News of his death sparked violence among angry residents of his southwestern Yoruba heartland around the commercial capital Lagos.

The ruling military council was due to meet on Wednesday to discuss the situation and a new transition to civilian rule.

Abiola's death in the inland capital Abuja came barely four weeks after a heart attack killed General Sani Abacha, the man who locked him up in 1994 for defiantly laying claim to the presidency.

It also came on the eve of an expected announcement of a new plan to restore democracy by military ruler General Abdulsalam Abubakar, whose regime promised a post mortem to clearly establish the cause of death.

Hundreds of Lagos residents attacked cars and smashed shops belonging to people from northern Nigeria -- home to the country's politicized generals. Police later brought the situation under control.

Abiola, a 60-year-old Muslim tycoon, was seen by political analysts as the key to end the political crisis which has bedeviled Africa's most populous nation since the annulment of 1993 elections he was poised to win.

His release had been widely anticipated after United Nations Secretary-General Kofi Annan suggested he had agreed to drop his presidential claim.

Resolving the crisis over the annulled election was the biggest task facing Abubakar, who had set free at least 30 political prisoners and tried to patch up relations with Nigeria's old friends abroad, who had shunned Abacha for human rights abuses.

The United States, whose envoy Under Secretary of State Thomas Pickering was with Abiola when he collapsed, said it had no reason to suspect that the death was caused by anything but natural causes.

``We have no indication that suggests that it was anything other than what it appeared to the (U.S.) team to be, that is cardiac arrest,'' said one U.S. official in Washington.

Nigerian officials said Abiola's personal doctor had been summoned to the clinic at the Aso Rock presidential villa, where the body lay, to be present at an autopsy if agreed to by the family.

Abubakar was due to give a speech on Wednesday to mark the end of an official month of mourning for Abacha and was also expected to lay out plans for a new transition program to democracy.

Officials at the presidency said it was likely the address would go ahead, but did not know what the general might say.

Hopes had been raised after Abacha's death that Abubakar, a softly-spoken 56-year-old career soldier, might be able to put Nigeria on a track to democracy. The country has enjoyed just 10 years of civilian rule since independence from Britain in 1960.

Pickering's visit was the latest sign of the rapid improvement in relations between Nigeria and the international community. It followed visits from Annan, Commonwealth Secretary-General Emeka Anyaoku and a British European Union envoy.

All of them had made plain they felt that Nigeria's best hope lay not in making Abiola president, but in organizing elections as quickly as possible in which anyone who wanted to could participate.

Abiola's death and fears of a backlash from his southwestern homeland raised new concerns about the country's future and one of the main Nigerian foreign-based opposition movements predicted there would be unrest.

``Let no one be in any doubt that there will be tragic consequences. A hero of the people has just died under incarceration. Do you think people are going to take this as another act of God?'' Professor Bolaji Akinyemi, spokesman for the NADECO-Abroad movement, told Reuters in London.

Muslim tradition stipulates that Abiola should be buried within 24 hours of his death and police cordoned off his home in Lagos to prevent it becoming a focal point for anti-government protests.

They declared an alert across the city of eight million where support for Abiola is strongest, but it was not clear if he would be buried there or at his birthplace in Abeokuta, some 40 miles away.

Abiola's supporters had demanded that he be made the head of a national unity government, but political analysts said it would have been impossible for Abubakar to sell that to his fellow officers, even if he had wanted to.

The June 12, 1993 election polarized ethnic rivals in Nigeria, which fought a civil war in the 1960s during which one million people died.