Raila Odinga, the opposition leader, accused election officials of handing a re-election victory to President Uhuru Kenyatta.
NAIROBI, Kenya — Tensions over Kenya’s presidential election built on Thursday after the opposition leader accused election officials of falsifying the results and handing victory to President Uhuru Kenyatta, the incumbent.
Raila Odinga, leader of the National Super Alliance, an opposition umbrella group, appeared unwilling to accept defeat, saying his party had received information from “confidential sources” within the electoral commission with data showing Mr. Odinga had won by about 300,000 votes.
Preliminary results that were live-streamed on television shortly after polling stations closed on Tuesday showed that Mr. Kenyatta, 55, was re-elected with 54.28 percent of the vote, easily surpassing the 50 percent threshold needed to avoid a runoff. Mr. Odinga, 72, received 44.83 percent of the vote. Under Kenyan law, the results are supposed to be certified by Monday.
On Thursday, Mr. Odinga said that he had won about 8.04 million votes and that Mr. Kenyatta had gotten 7.75 million.
The opposition leader’s claims, which came a day after he said that the voting system had been hacked, could further rile his supporters and tip the country into violence. Many of his supporters, who believe the election was stolen, are furious, and some have already clashed with the police in major cities.
Throughout his campaign, Mr. Odinga told supporters the election was rigged. He has said he was robbed of victory in the previous two contests. In 2013, Mr. Kenyatta won by a tiny margin, prompting Mr. Odinga to ask the Supreme Court to invalidate the election.
Mr. Odinga has called on his supporters to remain calm, but at a news conference on Wednesday he also said, “I don’t control the people.”
At the same news conference, Mr. Odinga’s running mate, Kalonzo Musyoka, issued what seemed to be a call for supporters to go out into the streets when they were given a signal. “There may come a time when we need to call you into action,” he said.
Andrew Limo, a spokesman for the Independent Electoral and Boundaries Commission, rejected Mr. Odinga’s latest claims. “The results come from the election management body,” he said by telephone. “They cannot come from a candidate or party.”
International observers have widely applauded the electoral commission’s conduct, and note that the official results are based on paper ballots tallied and verified at polling stations, not those transmitted electronically.
There has been no evidence so far that votes have been tampered with at polling stations or that the results contradict reports from party agents present during the counting of ballots, said John Kerry, the former United States secretary of state and an election observer for the Carter Center.
“So obviously there is a moment of reckoning coming up pretty soon,” he said in a telephone interview. “And the reckoning will be based on a paper trail.”
The electoral commission has already received nearly all documents detailing the results from each polling station, Mr. Kerry said. “The process is very meticulous and, plus, all the parties have copies of the documents,” he said. “So there is a huge trail of documentation. All of that will ultimately give confidence to people that they can trust in the outcome of the vote.”
Another top Western observer, speaking on the condition of anonymity because observers do not want to be seen as interfering, cast doubt on Mr. Odinga’s assertions. She said she had seen no evidence of vote-rigging.
The new claims by the opposition came a day after Mr. Odinga said that election commission servers had been hacked to award Mr. Kenyatta a 10-point lead. Mr. Odinga described the 2017 election a “fraud.”
The hackers, he told reporters earlier this week, had used the credentials of an election official who was killed days before the vote to gain access to the servers and then doctor the results. “They loaded an algorithm which is a formula to create a percentage gap of 11 percent between our numbers,” he said.
The electoral commission says hackers tried but failed to break into the server. “There were no external or internal interferences with the system at any point before, during and after the voting,” said Ezra Chiloba, commission’s chief executive.
Confusion and anxiety surrounding the election have worn out some Kenyans, who are staying home to avoid potential violence, even though the vote was carried out peacefully. Shops remain closed and roads in Nairobi, usually clogged with traffic, were empty on Thursday.
“I’m so tired of all this,” Kevin Otieng, 26, said wearily. “At every election, things always go wrong.”