LAGOS, Nigeria (AP) _ Preliminary findings of an independent autopsy Saturday indicated that Nigeria's most prominent political prisoner died naturally of a heart attack. The findings were released just after Moshood Abiola was buried in a garden ceremony at his Lagos home.

``In our opinion the mechanism of death is due to a rapid deterioration in a diseased heart,'' Canadian Dr. James Young told reporters. ``At this time, our preliminary opinion is that death was due to natural causes as a result of his long-standing heart disease.''

Young, the chief coroner in the Canadian province of Ontario, was one of several forensic scientists flown to Nigeria to conduct the autopsy.

The team's findings could help ease tensions in Nigeria, although Dr. Ore Falomo, Abiola's personal physician, said that a full autopsy report would be issued only after additional forensic tests on body tissue samples that were to be flown to Britain and Canada.

Abiola died Tuesday in custody while meeting with a group of visiting U.S. officials. He had languished in prison for four years, although his release had been expected soon.

Shortly after his death, Abiola's family said they suspected foul play. The timing of his death _ a month after dictator Gen. Sani Abacha's demise _ raised a number of eyebrows and spawned conspiracy theories. Rioting and violence erupted in Lagos and elsewhere in southern Nigeria, where at least 55 people have been killed and more than 400 arrested.

The Nigerian government already had concluded its autopsy on Abiola; several of its doctors attended Saturday's procedure, Falomo said.

The independent autopsy had been delayed until the weekend at Falomo's request. He did not explain the delay, although he may have wanted to wait until several foreign experts arrived.

Abiola's family offered no immediate reaction to the results of the autopsy, but one of his daughters, Wura, appealed for calm.

``We are in pain,'' she told a crowd of supporters gathered at their house. ``You are in pain, everybody is in pain, but I am pleading that _ as a peace-loving person _ our father would not like to see violence at this occasion.''

About 20,000 people gathered in the drizzle, crowding the streets in front of Abiola's home, to say goodbye to their hero.

His many admirers gathered near the compound's entrance gate; some wept, others prayed, but virtually everyone in the huge crowd fell silent during the service.

``It's so sad, it's such a sad day for Nigeria,'' said one mourner who would not give his name. ``How are we going to fill the void Abiola has left?''

An army of riot police and soldiers who had surrounded Abiola's residence before the funeral pulled back to more than a mile away in a rare act of subservience to the demands of the crowd.

Outrage at Abiola's death has begun to fracture the country along ethnic lines. Abiola, an immensely popular southern businessman from the Yoruba clan, posed a serious threat to the ruling military's stranglehold on power. In Nigeria, the upper echelons of power are controlled by conservative northern Muslims from the Hausa clan.

The bodies of nine Hausas were discovered earlier in the week in a predominantly Yoruba neighborhood. Fearing more of the same, riot police continued to watch over the city's mosques Saturday. The streets were, however, relatively calm.

The Hausa-Yoruba enmity was born after Nigeria's 1960 independence from Britain. Through their influence in the military, Hausa and closely related Fulani northerners have ruled Nigeria.

Southerners, both ethnically and religiously different from their Hausa and Fulani compatriots, long have resented what they consider the exploitation of their resources _ oil and other minerals _ by northern rulers.

These days, Nigeria's pro-democracy movement is largely led by southerners, particularly Yorubas.

Since Abacha's death on June 8, the new military junta has been scrambling to gain public trust. Nigeria's new military leader, Gen. Abdulsalam Abubakar has released a group of political prisoners and promised to free the rest. He also has vowed _ albeit vaguely _ to return the country to civilian rule.

Meanwhile, South African Deputy President Thabo Mbeki said Abubakar seeks to transfer power to a civilian government and is contemplating the composition of a government of national unity.

Mbeki spoke in Pretoria, South Africa, Saturday, shortly after returning from a two-day trip to Nigeria, where he met with Abubakar.

``We are fully convinced that a very important window of opportunity exists in Nigeria to move the democratization process forward,'' Mbeki said.