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.
Mr Odinga earlier told journalists the details were "fake", because the authorities had failed to present documents verifying the results.
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.
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.
He said that the results were "the work of a computer" and did not reflect the will of voters.
"We have our projections from our agents which show we are ahead by far," he added.
Opposition officials have said that, despite assurances from the electoral commission, they still have not received the result forms.
What does the electoral commission say?
The electoral commission has been urging people to wait calmly for the full results of Tuesday's vote.
"During this critical phase, we urge all Kenyans to exercise restraint as we await official results from the polling stations and indeed as they start trickling in," the commission said.