''The federal government regrets to announce the sudden death of Chief M.K.O. (Moshood) Abiola,'' a government statement said.

''Chief Abiola was taken ill during a meeting which was being held by Nigerian and United States officials with him,'' the statement said.

The statement from the president's office said an autopsy would be performed.

In Washington, a senior State Department official confirmed that Mr. Abiola fell ill while meeting with a delegation led by Undersecretary of State Thomas Pickering. The State Department official, asking not to be identified, said Mr. Abiola began coughing and wheezing and apparently died of a heart attack.

Mr. Abiola was reportedly to have been freed soon after four years of imprisonment.

One of only two candidates in an election in 1993, judged by the international community to be free and fair, he became the figurehead of the opposition after the election was voided by the military.

After the military crushed protests against the cancellation of the vote, Mr. Abiola announced in 1994 that he considered himself the rightful president.

He was almost immediately arrested on treason charges by the authorities under the then ruler, General Sani Abacha. General Abacha himself died June 8 of a heart attack.

Kofi Annan, UN secretary-general, visited Nigeria last week and pushed for Mr. Abiola's release.

Mr. Abiola's family had repeatedly warned that his health had been failing after years in detention under harsh conditions. His fortune and personal life also suffered during his imprisonment.

Two of his many wives died - one in an assassination-style slaying - and his business empire largely collapsed.

Mr. Annan said after his visit that he met with Mr. Abiola and that the opposition leader had decided to relinquish his claim to the presidency and cooperate with Nigeria's junta in making a peaceful transition to democracy. Many of his supporters, however, still wanted to see him take office.

Nigeria became an international pariah under the rule of General Abacha, who took power in the West African nation following a coup in 1993.

Political jailings were rampant; between 250 and 280 people are believed still held across Nigeria. All are kept in regular prisons except for Mr. Abiola, who was detained in the relative comfort of a government guest house in Abuja, the capital.

During General Abacha's years in power, corruption tore away at Nigeria's economy and its infrastructure, leaving both in shambles. Despite the country's vast oil reserves, motorists line up and wait for hours at the few gas stations that have not run dry.

An unlikely symbol for the opposition, Mr. Abiola was a millionaire businessman from the southwest Yoruba region.

An accountant by training, he worked for a while as West African representative of the American telecommunications giant ITT before branching out into business for himself.

With wide interests, from fishing to transport to the media, he made himself a dollar millionaire.

Married with four wives and a large number of children, he was assured during his captivity of their vocal support. On June 4, 1996, however, one of his wives, Kudiratu Abiola, was shot to death. The killers were never found, and there was speculation on the involvement of security officials.