Abiola's death at age 60 came after he became ill during a meeting with U.S. officials, who were visiting to discuss freedom for all of Nigeria's political prisoners, which number at least 250. The Nigerian government said he died of a heart attack.

Abiola was the apparent winner of 1993 presidential elections, which were annulled by the military government then in power. Gen. Sani Abacha, who took power in a coup later that year, jailed Abiola in 1994, accusing him of treason.

Abacha died last month of a heart attack.

The government said an autopsy would be done, with the consent of Abiola's family and with his doctors participating, to determine the cause of death.

Abiola's family repeatedly had warned that his health was failing after years in detention under harsh conditions.

"We were filled with such hope that my father would finally come home to us," she said. "My father was a great man that believed in his country, and we know that this is very suspicious.

"I think that we must keep our eyes on the issues here. The issue is the military held my father in incarceration and he died while in their care."

A wealthy businessman from southwest Nigeria, Abiola had ties to the military when he ran for president in 1993. But he was running against a candidate the military favored more.

When it appeared that Abiola was winning, the government annulled the election. In June 1994, Abiola marked the first anniversary of the election by declaring himself the winner.

Abacha had Abiola thrown in prison for making the claim.

After Abacha's death in early June, Gen. Abdulsalam Abubakar succeeded him. Abubakar appears to want to have little to do with Abacha's policies.

In just a few weeks, he has freed dozens of political prisoners, met with prominent international delegations and vowed to hold elections and hand over power to civilian rule.

There had even been reports that Abiola would be freed this week, following the 30-day mourning period for Abacha that ended Tuesday.

"I walked in and shook hands and he asked what happened to the Egyptian," Annan said, in reference to his predecessor Boutros Boutros-Ghali, who left office in December 1996.

He said he has spoken to Abubakar and hoped the government "will make good on its pledge to release all remaining political prisoners, unconditionally, and to define a credible process for the democratic transition to civilian rule within a reasonable time frame."

Undersecretary of State Thomas Pickering told CNN that Abiola appeared clear and lucid at the start of the meeting, but that partway through the meeting, he asked for tea and had trouble breathing. Later, Abiola left the room.

Pickering said that when Abiola returned, he was distressed, and asked for cough medicine and pain killers. He sat on the couch, removed his shirt and asked that the room be ventilated.

Pickering said speculation about the cause of Abiola's death "is inappropriate at this time."

"My mission here is to seek the release of all political prisoners including Abiola," Pickering said. "That's the direction in which they appeared to be moving when this untimely event intervened."

It was unclear whether Abiola's death would affect the new government's stability.

"It's shocking," said Roman Catholic Archbishop of Lagos Anthony Okogie. "His death is the end of a chapter."

Abiola first won international attention in 1992, when he began a campaign to get the United States and former European colonizers of Africa to pay reparations to the continent for slavery and for fortunes made from exporting raw materials.

Babangida feared that a victory for Abiola, a member of the southern Yoruba tribe, would threaten northerners who have long dominated the military and politics in Nigeria.

He amassed a fortune in publishing, shipping and oil, but said his first serious income came when he worked for the International Telephone and Telegraph Corp., serving as a vice president and chairman for Africa and the Middle East from 1971 to 1988.

Some Nigerians blamed him for the country's appalling telephone system, saying he made millions by using inferior materials and pocketing the extra money, as many have in corruption-ridden Nigeria. Abiola denied the accusations.

Abiola's long prison stay is said to have devastated his financial empire, leaving many of his businesses struggling to survive.

Abiola's personal life kept him in the public eye before he got into politics.

In 1992, a New Jersey judge ordered him to pay $20,000 a month in child support to a woman who said he was his wife. Abiola's lawyers argued that he had four wives and that the woman was just one of his 19 concubines.

His biography in "Who's Who in Nigeria" notes that he "has many children."

One of Abiola's wives, Kudirat, was gunned down in 1996 in a roadside ambush that also killed her driver. She was an outspoken critic of the Abacha regime, which was blamed by opponents for her killing. Police said it was a result of a family feud.