A Nigerian government statement said he died of a heart attack.

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

Gen. Abacha died last month of a heart attack. His five-year rule made Nigeria an international pariah.

In Washington, President Clinton expressed regret at Mr. Abiola's "sudden and untimely" death. Mr. Clinton said the visiting U.S. officials saw Mr. Abiola fall ill and watched Nigerian doctors' last-ditch efforts to save his life at a government clinic.

A State Department official said there was no reason to believe Mr. Abiola died of anything but natural causes. The official, who spoke on condition of anonymity, said Mr. Abiola began coughing and wheezing and apparently died of a heart attack.

The U.S. delegation, led by Thomas Pickering, former ambassador to Nigeria, was in the country to meet with the new military leader, Gen. Abdulsalam Abubakar, and to lobby for the release of political prisoners.

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

Mr. Abiola became a rallying point for Nigeria's disparate opposition groups during his years of imprisonment.

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

Last week, U.N. Secretary-General Kofi Annan visited Nigeria and announced that Gen. Abubakar was planning to release Mr. Abiola and other political prisoners soon.

There had even been reports that Mr. Abiola would be freed this week, after a 30-day mourning period for Gen. Abacha, which ended Tuesday.

American-Nigerian relations have been poor in recent years, particularly after Gen. Abacha executed playwright and pro-democracy activist Ken Saro-Wiwa and eight other people in November 1995.

But since taking power, Gen. Abubakar has surprised and pleased the United States by releasing some political prisoners and pledging that all eventually will be freed.

American companies purchase millions of dollars in Nigerian oil each year.

Mr. Abiola grew up in a poor family and earned a degree in economics from the University of Glasgow in Scotland. He later amassed a fortune in industries including publishing, shipping, and oil.

He worked for the International Telephone and Telegraph Corp., serving as company 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. Mr. Abiola denied the accusations.

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

One of his wives, Kudirat, was gunned down in 1996 in a roadside ambush that also killed her driver. Opponents of Gen. Abacha blamed his government for the killing but police said it was a result of a family feud.