Kenya's opposition presidential candidate, Raila Odinga, on Tuesday rejected the tally from this week's election, saying hackers had manipulated data in the election commission's computer system.
At least three opposition supporters were killed in protests that erupted in several areas after Odinga made his accusation, according to witnesses and police.
The results of the hotly contested campaign have not been announced, but early results posted by the Independent Electoral and Boundaries Commission give President Uhuru Kenyatta a strong lead.
Commission chief Wafula Chebukati said Wednesday he did not know whether the system had been hacked but vowed to investigate the claim. He said the progressive tally on the website was not the final result and promised an audit of voting before a final announcement.
With about 95% of the vote counted, the president has opened up a lead of 1.4 million votes, according to the count recorded on the commission website. Odinga's rejection of the tally fueled tensions among opposition supporters fearful of a stolen election, and raised fears a disputed result could trigger violence.
The opposition claim centered on the torture and slaying of a key election official, Chris Msando, who was in charge of information and communications technology at the election commission, just days before the vote. Information technology experts for the opposition claimed Wednesday that Msando's user ID and password had been used to hack into the IEBC computer system and to gain access to Chebukati's account starting at 12:37 a.m. They said an algorithm had been planted that gave Kenyatta an automatic 11% lead.
"This is a fraud of monumental gravity. Some people conspired and denied Kenyans the opportunity to elect their leaders," Odinga said. "We didn't have an election."
He said the opposition's own informal vote tally gave him 8.1 million votes compared with 7.2 million for Kenyatta. He released 50 pages of what he claimed were the computer logs of the electoral commission which he said proved his claims.
Elections in Kenya are ethnically charged, and past presidential elections have been flawed, according to international observers. With tensions as high as they are, there are fears of a repeat of the ethnic killings that followed the disputed 2007 election, which left up to 1,500 people dead.
Opposition supporters reacted angrily to Odinga's claims, although he urged restraint.
"We have urged people to remain calm as we delve deeper into this matter. I don't control the people but I have asked them to remain calm," he told journalists.
But witnesses said at least two protesters were shot dead by police in the Mathare slum district after angry opposition supporters took to the streets, chanting "Uhuru must go!" and throwing rocks.
One witness, Alice Adhiambo, 40, said she saw one young male protester shot in the head by police. She said the demonstrators confronted police in the afternoon but fled when they fired tear gas and bullets. Another protester was taken to a clinic after being shot in the leg, according to witnesses.
Earlier, furious opposition supporters in Mathare harassed passersby, demanding to know their ethnic group and robbing those from rival ethnic groups, according to a local peace activist, Jakiwa Inda.
"After Raila issued this statement, tension came very high. Most of the youths were outside and I saw youths have started to harass people who were just walking. They started asking, 'What tribe are you?'" Inda said community leaders had urged the angry opposition supporters to calm down without success.
"We are trying to plead with them to wait for the issue of the result to settle," he said.
Nairobi's police chief, Japheth Koome, confirmed two people were shot dead and said they were stealing, the Associated Press reported.
A regional police commander in Kisii county in western Kenya, Leonard Katana, confirmed that police opened fire on opposition protesters, killing one person, the AP reported.
Chebukati said the electoral commission had called for an examination of the official tally papers from more than 40,000 polling stations around the country and would base the final result on the forms.
"As for now, I cannot say whether the election has been hacked. We shall go into that and find out whether those claims are true.
"We just want to comply with the law. But we have had concerns raised and we can't ignore those concerns. At the end of the day we shall do an audit and those questions will be answered," he said.
In Nairobi's Kibera slum, there was also tension over the opposition allegations of election hacking and fraud, according to witnesses.
"People are not happy," said opposition supporter Oyoo Ogango, 30, who is unemployed with three children. "Now I'm worried. At first we thought it was fair, but it has been revealed that some shoddy things have happened.
"Kenyans are good people. They just want to know that the election was fair. All we need is credible elections."
Ogango also feared that any future call from opposition leaders for demonstrations against the result could lead to mayhem in Kibera and other areas.
"When there's some doctoring, I must be worried. I'm a Kenyan. I went out and voted peacefully. What I was expecting was a credible election, a peaceful result."
Reuters reported that police fired tear gas in Kisumu, western Kenya, an opposition stronghold, to disperse people chanting, "No Raila, no peace."
Voting in 2007 was peaceful, but violence flared after Odinga, who was the opposition candidate at the time, rejected the result and the then-president, Mwai Kibaki, had himself sworn back into office in the dead of night. In the weeks that followed, members of Kibaki's Kikuyu ethnic group and Odinga's Luo group attacked, killed and raped one another and burned down houses. Other ethnic groups were also drawn into the violence.
Kenyatta, who is also Kikuyu, succeeded Kibaki as president in 2013 elections that Odinga also contested and disputed, claiming the government rigged the vote during electronic transmission of results to the central tally center.
Odinga took his challenge to court in 2013 and lost. He has not indicated how he will mount any challenge to the results in this election.
9:35 a.m.: This article was updated with reports of at least three opposition supporters killed.
6:20 a.m.: This article was updated throughout with staff reporting.