Black smoke billowed up over a large market in central Lagos Thursday from a blaze police say was set by youths enraged by the death of Nigeria's leading political prisoner.

Although Lagos was calmer after two tumultuous days of rioting, the Agege Market fire - and clashes between demonstrators and police in three other parts of the city - indicated life in Nigeria was far from returning to normal. No injuries were reported from Thursday's clashes and no further details were available.At least 19 people were killed in Wednesday's violence. Police used tear gas to quell the unrest.

In Washington, the State Department issued a travel warning late Wednesday for all U.S. citizens in Nigeria. "U.S. citizens are advised to avoid all unnecessary travel, especially by car, remain vigilant and avoid crowds," it said.

Police and firefighters battled the blaze that started midmorning in several market stalls. Several houses were reportedly also set alight by marauding groups of youths.

The arson and clashes were the latest tumult sparked by the death of Moshood Abiola, the country's top political prisoner and the man many hoped would become Nigeria's next civilian president. Abiola died Tuesday of what the government says was a heart attack.

Rioters who hold the government responsible for his death tore through parts of Lagos on Wednesday, as well as the southern university town of Ibadan and Abiola's hometown of Abeokuta.

In Lagos, many people stayed home Thursday, and the smoldering ruins of the mayhem - piles of burning tires - were scattered along the streets.

Adding to the confusion, Nigeria's new military leader dissolved his Cabinet on Wednesday. However, Gen. Abdulsalam Abubakar left untouched the Provisional Ruling Council, the core of the junta that rules this oil-rich but poverty-wracked West African nation.

The Provisional Ruling Council was to meet Thursday night to discuss plans to hand over power to civilians and to release Nigeria's remaining political prisoners. It also was expected to discuss the fate of those convicted earlier this year of plotting a coup against Nigeria's late dictator, Gen. Sani Abacha. Nigeria's former deputy ruler, Gen. Oladipo Diya, and five others were sentenced to die by firing squad.

Abubakar appealed for calm in a nationwide television address Wednesday night, calling Abiola's death was a "national tragedy."

"Our national grief cannot be assuaged by recourse to lawlessness," he said, extending his condolences to Abiola's family, friends and "to the many young men and women for whom he held out hope."

Abiola, who had spent the past four years in prison, died Tuesday after suddenly falling sick during a meeting with U.S. diplomats. He was 60.

He was a potent, if unlikely, symbol of reform for many Nigerians. A multimillionaire who was once close to the country's junta, he eventually turned against them and ran for president in 1993. He was the apparent winner of those elections, which were canceled by the military. He was arrested for treason and jailed a year later.

The government says Abiola died of a heart attack, but has gone along with demands by his family to have independent, non-Nigerian pathologists monitor the autopsy.

The death is the latest shock wave in a turbulent month for Nigeria that began June 8 when Abacha died, also of a heart attack. During his rule, Abacha was widely accused of corruption and extensive human rights violations.