Supporters of Kenyan opposition presidential candidate Raila Odinga march in Nairobi’s Mathare slum Thursday. (Marco Longari/AFP/Getty Images)

A day before Kenya is du e to announce the official outcome of its recent presidential election, the country’s longtime opposition leader on Thursday reasserted his conviction that the results have been rigged.

The National Super Alliance, the party led by Raila Odinga, declared that he had won Tuesday’s vote by a wide margin, even though provisional results indicate that President Uhuru Kenyatta won by a margin of nearly 10 percent. That message laid the groundwork for a potentially explosive reaction when the official results are announced Friday.

In a nation with a recent history of solid economic growth and relative political stability, Kenya’s elections nearly always expose the tense, fractured tribal politics that lie just under the surface here. In 2007, a disputed election led to about 1,400 deaths, and charges were brought at the International Criminal Court — including against Kenyatta and his running mate, William Ruto — that were dropped for lack of evidence.

This time, much will hinge on the direction Odinga, 72, gives his followers, who have already taken to the streets in periodic bursts, confronting police who have responded with tear gas and, less frequently, live ammunition. International election monitors, including former U.S. secretary of state John F. Kerry, have encouraged Odinga and Kenyatta to advocate a peaceful response by their parties and supporters.

But at a news conference Thursday, Odinga’s team described what it said was a vast coverup by the country’s Independent Electoral and Boundaries Commission, citing as its source an anonymous whistleblower inside the commission. According to Musalia Mudavadi, one of Odinga’s advisers, the “authentic and legitimate results” were manipulated or hidden by the body.

The commission’s unofficial results showed Kenyatta, 55, leading with 54.3 percent of the vote, ahead of Odinga’s 45 percent. By Odinga’s count, those figures would be roughly reversed.

Given Kenya’s wide political divisions, analysts worry that another disputed election will only increase tensions between ethnic groups and further diminish faith in the central government.

“If you have a leader that commands only half the country, you will have many disgruntled people and low confidence in public institutions,” said Murithi Mutiga, a researcher with the International Crisis Group.

On Wednesday, Odinga made a different set of allegations against the electoral commission, claiming that its system had been hacked and results were manipulated using stolen log-in credentials. On Thursday, the chairman of the commission, Wafula Chebukati, acknowledged that there had been a hacking attempt but said it “did not succeed.”

Later Thursday, after Odinga’s coalition announced its tally, celebrations broke out in the streets of Kisumu, the opposition’s western stronghold. There was also jubilation in Kibera, a vast slum in Nairobi, the capital, where young men watched Odinga’s news conference on television and listened on radios outside their homes.

The results announced by Odinga are “just the way we believe they should be,” said Dalmas Omondi, 28, who said he’d spotted a white police helicopter hovering over Kibera while he gathered with other Odinga supporters.

But residents across Nairobi and other major cities also braced for the prospect that the official figures Friday will differ from Odinga’s. That is the major concern on the minds of many Kenyans, as well as foreign diplomats and business executives, whose interest is in a peaceful resolution of the electoral dispute that has paralyzed the country since Tuesday’s vote. Businesses have been shuttered, and the streets have been uncharacteristically quiet.

“A lot will depend on the behavior of the losing candidate,” Mutiga said. “If he calls for sustained protests, we may be looking at a few days, and maybe weeks, of uncertainty.”

Odinga has not said clearly how he will respond if Kenyatta is declared the winner. In 2013, he took his complaints to the Kenyan courts, which eventually dismissed his case. This time — likely to be his last bid for the presidency — some worry he’ll take a more aggressive approach.

In Kibera, Odinga’s young followers call him “Baba,” or father, and say they will wait on his guidance after results are released Friday. “When he says jump, we will ask, ‘How high?’ ” said Gordon Odhiambo, 27.