Kenya opposition presidential candidate Raila Odinga has said the electoral commission's IT system has been hacked to manipulate the election results.
He rejected early results from Tuesday's vote indicating a strong lead for President Uhuru Kenyatta.
The electoral commission has not yet responded to Mr Odinga's accusation, but politicians have called for calm.
Mr Odinga said that the hackers gained access to the Independent Electoral and Boundaries Commission (IEBC) computer system by using the identity of the commission's IT manager, Chris Msando, who was killed last month.
The opposition leader had earlier told journalists the results coming in were "fake", because the authorities had failed to present documents verifying the results.
Electoral officials say that with 91% of results in, Mr Kenyatta is leading with about 54.5%, to Mr Odinga's 44.6%.
These results mean Mr Kenyatta appears to be heading for a first-round victory. In order to avoid a run-off, a candidate needs 50% plus one of the votes cast and at least a 25% share of the vote in 24 of Kenya's 47 counties.
There were eight candidates in all, but apart from Mr Kenyatta and Mr Odinga none polled more than 0.3% of the vote.
What is Mr Odinga's complaint about the vote?
The opposition has described the results being released online as a "fraud" because they were not accompanied by original result forms 34A and 34B from the polling stations.
"They are fictitious, they are fake," said Mr Odinga.
Many fear a repeat of the violence after a disputed election 10 years ago. More than 1,100 Kenyans died and 600,000 were displaced following the 2007 vote.