ABUJA, Nigeria (AP) - Nigeria's most prominent political prisoner, Moshood Abiola, died of an apparent heart attack Tuesday after suddenly becoming ill during a meeting with U.S. diplomats who came to push for his freedom.

Abiola's death came just as the end to his four years' imprisonment appeared imminent, and was the latest shock in a turbulent month for Africa's most-populous nation. The dictator who had jailed him died June 8, of a heart attack.

The news prompted demonstrators Tuesday to vent their fury in the streets, throwing rocks and burning debris in the roads. A former backer of Nigeria's military regime who later ran on his own for president on an anti-poverty platform, Abiola became a hero of pro-democracy activists.

A U.S. State Department official said Abiola began coughing and wheezing during a meeting with a delegation led by U.S. Undersecretary of State Thomas Pickering.

Pickering said Abiola became ill in the early stages of their meeting and asked for a doctor.

``We all helped to put him in a car. There was no ambulance immediately available,'' Pickering said in an interview with CNN.

Abiola had been drinking tea and chatting before he fell ill, said Jim Callahan, a spokesman for the U.S. Embassy in Nigeria.

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

A Nigerian government statement said the cause of death was a heart attack, and the State Department said there was no reason to believe that the death was not the result of natural causes. Abiola's family had repeatedly warned that his health had been failing after years in detention under harsh conditions, but cast doubt on the official explanation of his death.

His daughter, Wuru, fighting back tears in a British Broadcasting Corp. TV interview, said that ``of all the conditions he had, heart was not one of them.''

Another daughter, Hafsat, said in an interview with CNN that the timing was suspicious.

``It was too convenient,'' she said. ``All of a sudden at the eve of his release, he dies.''

Abiola was the apparent winner of the 1993 presidential elections that were canceled by the military. He was jailed the following year and accused of treason by dictator Gen. Sani Abacha, who died last month of a heart attack.

Dissidents also have complained that U.N. Secretary-General Kofi Annan and other diplomats refused to stand up for Abiola - and said the secretary-general pressured Abiola into renouncing his claim to the presidency.

Annan also was criticized for not going to Lagos, Nigeria's commercial center where much of the country's opposition movement is centered. Annan did meet with opposition groups in Abuja, the capital.

The U.S. delegation did not plan to visit Lagos either. The delegation was to hold a small meeting with civilian leaders, including some opposition figures, said Charles Patterson, the principal officer at the U.S. Embassy in Abuja.

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

But since taking power, the new military leader, Gen. Abdulsalam 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 oil from Nigeria, one of world's leading producers with an output of 2 million barrels a day.

An unlikely hero for democracy, Abiola became a rallying point for Nigeria's disparate opposition groups during his years of imprisonment.

Grief and sorrow swept across the country as news of his death spread among his supporters.

In the commercial capital of Lagos, news of Abiola's death was met with isolated incidents of stone throwing.

Just north of Lagos, however, in the university town of Ibadan, students took to the streets in a show of outrage over Abiola's death while still in military custody.

More than a thousand students blocked key intersections and set enormous bonfires along the roads, a police spokesman said. There were no immediate reports of arrests or injuries linked to the protests.

In New York, Annan, called on Nigeria's new military leader to fulfill promises to release all political prisoners and ``define a credible process for the democratic transition to civilian rule.''

British Foreign Secretary Robin Cook said Nigeria had lost its ``symbol of democracy.''

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.