Riots rocked Nigeria's largest city Wednesday, killing at least 10 people as protesters devastated over the death of the country's most prominent political prisoner clashed with police.

The family of Moshood Abiola, a potent symbol of reform for Nigeria's poor masses, refused to accept the official line that he died Tuesday of a heart attack.Meanwhile, the country's military ruler dissolved his Cabinet Wednesday. Gen. Abdulsalam Abu-ba-kar had inherited the Cabinet from his predecessor, dictator Gen. Sani Abacha, who died last month.

The Provisional Ruling Council, the core of Nigeria's ruling junta, was left intact.

Grief and anger swept across this West African nation - Africa's most populous - as news spread of Abiola's death.

In Lagos, thousands of youths and students staged tumultuous demonstrations along busy streets, setting tires on fire and throwing stones. Police used tear gas to disperse the crowds. The rioting appeared to be waning by midafternoon.

Schools were closed in Lagos state through Friday because of the violence.

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

Col. Mohammed Marwa, the popular military administrator of Lagos state, appealed for calm in a radio broadcast Wednesday.

"Violence will not profit anyone," he said. "Only God knows why he has taken Abiola away at this crucial period of national reconciliation."

Abiola, who was reportedly to be released soon 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.

After becoming ill, 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.

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. 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.

The U.S. State Department said there was no reason to suspect foul play. Abiola's family had repeatedly warned 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 Wednesday, 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 Wednesday that Nigeria's military leadership was warned just weeks ago that Abiola was in dire need of treatment.

Dr. Charles Clements, chairman of Physicians for Human Rights, sent the warning in a June 22 letter sent to Abubakar, the new Nigerian leader.

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.

U.N. Secretary-General Kofi Annan visited last week and announced 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.

In London, the pro-Conservative Daily Telegraph noted Wednesday that Nigeria has been under military rule for all but 10 years since its independence from Britain in 1960.

"The lion's share of the mess in which it finds itself . . . must be laid at the military's door," the newspaper said.