NAIROBI, Kenya — Hundreds of people stoned cars and rival ethnic groups fought in a sprawling slum Saturday amid tensions over delayed results from the closest presidential election in Kenya's history, witnesses said.

A millionaire opposition leader who cast himself as a champion of the poor appears poised to win the race, but only partial and unofficial tallies have been released from Thursday's vote.

In the Kibera slum, the main constituency of opposition candidate Raila Odinga, young men with machetes were running through the maze of potholed tracks and ramshackle dwellings. People set up makeshift roadblocks.

About 20 miles outside Nairobi, hundreds of people were massed along a main highway.

"They are looting houses and stoning cars," Irungu Wakogi, a witness, told The Associated Press by telephone.

The race pits President Mwai Kibaki against his former ally, Odinga, and marks the first time an incumbent has faced a credible challenge in Kenya's four decades of independence from Britain. The race focused largely on corruption, with both candidates vowing to end the graft and tribal favoritism that has tainted politics here for years.

A string of Kibaki's allies in parliament also were being unseated in the vote.

Kenyans clustered around radios and televisions as results trickled in, but by early Saturday the electoral commission had announced preliminary results in only 99 of Kenya's 210 constituencies.

Those counts, which show Odinga with a lead of 2,308,959 to Kibaki's 1,938,361, still must be certified. Unofficial results by local media, taken from tallies at some polling centers, also put Odinga in the lead.

Still the slow pace of official returns raised fears of rigging.

Feeling confident of a win, Odinga's Orange Democratic Movement party said the government was deliberately delaying results because they were losing. Police appealed for calm as tempers flared.

"We'd like the ECK (Electoral Commission of Kenya) to announce the results in order to ensure that the political temperature does not go up," said Joseph Nyagah, an ODM official.

U.S. Ambassador Michael Ranneberger said he had confidence in the electoral commission.

"A lot of time there are delays for general reasons," he said. "I think it is too early to comment on that or express particular concern."

Constance Newman, head of an American observer group, said the official process was cumbersome, likening it to "molasses in winter." The European Union observer team said there was no immediate evidence of rigging.

Kibaki, 76, has been credited with helping boost this East African nation's economy, with a growth rate that is among the highest in Africa and a booming tourism industry. But his anti-graft campaign has largely been seen as a failure, and the country still struggles with tribalism and poverty.

Odinga, a flamboyant 62-year-old former political prisoner, promised change and help for the poor. His main constituency is Kibera, one of Africa's largest slums, which is home to at least 700,000 people.

But he has been accused of failing to do enough to help his constituents during 15 years as a lawmaker.

Violence was a major concern in the run-up to the election, and several diplomats have expressed concern that a narrow victory on either side could lead to rioting by those who do not accept or trust the results. But the voting was generally orderly, and no major disruptions were reported.

Police Commissioner Mohamed Hussein Ali on Friday urged whoever wins "not to engage in unbridled celebrations that will cause resentment."

To the losers, he instructed, "take it with dignity."

Kibaki appealed for Kenyans to wait for official results.

In the run-up to the election, clashes in western Kenya killed hundreds. An outlawed gang called Mungiki that circulated leaflets in July calling on Kenyan youth to rise up against the government was blamed in a string of beheadings.

On Wednesday, authorities said opposition supporters had stoned three police officers to death in western Kenya, accusing them of being part of a government conspiracy to rig the elections.

If Kibaki loses, he will be Kenya's first sitting president ousted at the ballot box.

Kibaki won by a landslide victory in 2002, ending 24 years in power by Daniel arap Moi, who was constitutionally barred from extending his term. Moi's blanket use of patronage resulted in crippling mismanagement and a culture of corruption that plunged Kenya into an economic crisis.

Voters also were electing 210 members of parliament and those results also suggested Kenyans were aching for change. They kicked out some of the nation's political old guard, including the vice president and three key Kibaki allies linked to Anglo Leasing, the largest financial scam under the president's term.

Also voted out was 2004 Nobel Laureate Wangari Maathai, whose popularity as a politician never caught on here.