0123 GMT March 22, 2018
Police engaged in running battles with a few hundred protesters in Odinga's bastion Kisumu in western Kenya, firing tear gas as his supporters set tires alight. Burning barricades also went up in Nairobi's Mathare slum, AFP reporters said.
With ballots from 94 percent of polling stations counted, electoral commission (IEBC) results showed Kenyatta leading with 54.4 percent of the over 14 million ballots tallied against Odinga's 44.7 percent.
"These results are fake, it is a sham. They cannot be credible," Odinga told a press conference in the early hours of the morning as partial results fell quickly via an electronic tallying system aimed at preventing fraud.
His accusations, and the reaction from his supporters again raised the specter of electoral violence in Kenya, still traumatized by the memory of bloody post-poll clashes a decade ago which left 1,100 dead and 600,000 displaced.
Odinga later detailed accusations of a massive hacking attack on the electronic system, saying hackers had gained entry to the system using the identity of top IT official Chris Msando, who was found murdered and tortured late last month.
"This is an attack on our democracy. The 2017 general election was a fraud," said Odinga, claiming detailed evidence of the hacker's movements. He would not say how he got the information, as he wanted to "protect his source".
Odinga claimed the IEBC had not provided the scanned forms meant to accompany the results.
The 72-year-old, who is making his fourth bid for the presidency as the flagbearer for the National Super Alliance (NASA) coalition, accused his rivals of stealing victory from him through rigging in 2007 and in 2013.
"You can only cheat a people for so long," he said.
IEBC chairman Wafula Chebukati insisted the poll was "free and fair" and urged Kenyans to await the original forms backing up the results being streamed through electronically, to verify the figures seen on their public website.
"As a commission we shall carry out investigations to establish whether or not the (hacking) claims are true," he said.
Odinga urged his supporters to "remain calm as we look deep into this matter." But he added: "I don't control the people."
But police said officers opened fire on people protesting election results in an opposition stronghold in southwestern Kenya, killing one person.
Leonard Katana, a regional police commander, said the shooting happened on Wednesday when protesters clashed with security forces in South Mugirango constituency in Kisii County.