``Mobs of youths have been roaming the streets of the Mushin area destroying the shops belonging to northerners,'' said a witness in one section of the city of eight million. Police were ordered on city-wide alert and disturbances petered out in late evening.

Abiola died in Nigeria's inland capital Abuja barely four weeks after a heart attack killed General Sani Abacha, the man who locked him up.

The military government, under new leadership and seeking a fresh start with Western friends alienated by Abacha's human rights record, said 60-year-old tycoon apparently died of a heart attack. It promised that his family doctors would be allowed to examine his body.

Officials in Washington said there was no reason to doubt that the death was from natural causes. President Bill Clinton said he was deeply saddened by Abiola's ``sudden and untimely death.''

His daughter Hafsat Abiola told Reuters Television that the country's military rulers were to blame -- whether his death resulted from medical neglect during his years in prison or some more direct act like poisoning.

A self-made Muslim businessman who won wide popularity as sponsor of a successful soccer team, Abiola was leading the count in 1993 when the general then in control, Ibrahim Babangida, called an abrupt halt to the election he had promised would return Nigeria to the control of a civilian president.

He had suffered poor health since he was detained in 1994 for defiantly laying claim to the presidency.

Abacha's successor, General Abdulsalam Abubakar, pressed by Nigeria's Western friends to get the country on track to democracy, has freed a number of political prisoners and was widely expected to order Abiola's release.

But the move was delayed during a series of diplomatic comings and goings with the ruling generals insisting that not only Abiola but also his followers, who are mainly in the Yoruba-speaking southwest, abandon his claim to the presidency.

U.S. Under Secretary of State Thomas Pickering was present when Abiola collapsed and died. He immediately extended his stay.

An official statement from the presidency said Abiola died of an apparent cardiac arrest at 4 p.m. (1500 GMT) during a meeting with Nigerian officials and members of a United States delegation.

``With the consent of his family, the Federal Government has ordered a full post-mortem examination to ascertain the actual cause of death with the full participation of his personal physicians,'' the statement said.

But one of the main Nigerian foreign-based opposition movements predicted there would be unrest in the powerful African nation after Abiola's death.

``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.

And in Lagos, Nigeria's biggest city, witnesses said people screamed and shouted in the central business district and many rushed to get to their homes as night fell.

``They have killed Abiola. There will be trouble in the country,'' shouted one youth as he tore along Broad Street near the Reuters office in central Lagos.

Resolving the crisis over the annulled election was the biggest task facing Abubakar, who 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 suggestion last week by Kofi Annan that Abiola had given up his presidential claim provoked opposition anger..

Abiola's supporters demanded that he be made the head of a national unity government, but political analysts said that 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.

Abiola was from the Yoruba southwest. Abacha, Abubakar and Babangida were all from the north, like most of Nigeria's rulers since independence from Britain in 1960.

``This is terrible news. There is bound to be a great deal of suspicion,'' Olisa Agbakoba, leader of the opposition United Action for Democracy group told Reuters in Lagos.

Abubakar had been expected shortly to announce a new transition to civilian rule to replace the discredited scheme of Abacha, who had planned to stand unopposed in presidential elections.

The style of softly-spoken Abubakar, 56, is in marked contrast to Abacha's iron-fisted approach and has won him greater acceptance both in Nigeria and abroad.

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.

The ruling military council was due to meet on Wednesday to discuss the matter of Abiola and the question of a new transition to civilian rule.