ABUJA, Nigeria (AP) _ Police used tear gas today to disperse demonstrators angry over the death of Nigeria's most prominent political prisoner, while the man's family refused to accept the official line that he died of a heart attack.

Authorities said at least 10 people were killed in overnight clashes between protesters and police in Lagos.

Grief and anger swept across this West African nation today _ Africa's most populous _ as news spread of the death of Moshood Abiola, a potent symbol of reform for Nigeria's poor masses.

In Lagos, thousands of youths and students staged tumultuous demonstrations along busy streets, setting tires on fire and throwing stones. Gunshots rang out in parts of the city, but it was unclear who was shooting.

Students in the southern university town of Ibadan also staged protests.

Abiola, who reportedly was on the cusp of being released from four years of imprisonment, took ill Tuesday during a meeting with members of a U.S. delegation led by Undersecretary of State Thomas Pickering.

``Chief Abiola started talking with Undersecretary Pickering when he started feeling uncomfortable,'' said Jim Callahan, a spokesman for the U.S. Embassy in Nigeria. ``He was clearly in distress.''

Before he began coughing and wheezing, Abiola had been drinking tea and chatting with Pickering, Callahan said in a telephone interview.

Abiola, who grew up poor and later amassed a fortune in industries including publishing, shipping and oil, was an unlikely hero for Nigeria's nascent democracy movement.

Once an ally of Nigeria's all-powerful military, Abiola fell out of favor after winning 1993 presidential elections, which were annulled by the military government. Gen. Sani Abacha came to power in a coup later that year, and when Abiola insisted on his claim to the top office, Abacha jailed him and convicted him of treason.

After becoming ill during Tuesday's meeting, the 60-year-old Abiola was rushed to the hospital, where he was pronounced dead. The government said he had died of a heart attack and promised a full autopsy.

The U.S. State Department said there was no reason to suspect any foul play. 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 ``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.''

The family said Abiola was to be buried later today, but it was likely to be postponed until the autopsy could be performed.

Abiola's physician, Dr. Ore Falomo, said he was unable to participate in the autopsy and called for physicians from Britain and Canada to take part.

Meanwhile, a U.S.-based human rights group said today that Nigeria's military leadership was warned just weeks ago that Abiola was in dire need of treatment.

``Chief Abiola's American doctor, Dr. Mitchell Feinman, has concluded that his patient suffers from life-threatening conditions, which, if left untreated, could be fatal,'' said Dr. Charles Clements in a June 22 letter sent to the new Nigerian leader Gen. Abdulsalam Abubakar.

Abiola's death came on the same day Nigeria completed a 30-day official period of mourning for Abacha, who died of a heart attack last month following a five-year rule that left the oil-rich country impoverished and internationally scorned.

Sequestered for four years in a government guest house in Abuja _ most of the country's roughly 250 political prisoners are detained under far more wretched conditions _ Abiola was reportedly nearing freedom.

U.N. Secretary-General Kofi Annan visited last week and announced that the new military government would release all its political prisoners, including Abiola. Pickering's delegation had come to Nigeria to meet with the military leadership to push that process along.

Annan said following his visit that Abiola had agreed to relinquish his claim to the presidency and cooperate with the military junta. The U.N. chief said Abiola appeared to be in good health.

In Washington on Tuesday, President Clinton said Pickering's delegation accompanied Abiola to the hospital after he collapsed, ``and witnessed physicians at the State House clinic work to try to save Chief Abiola.''