With 85% of polling stations reporting results by the early hours of Wednesday morning, incumbent President Uhuru Kenyatta was seen to be leading his main challenger Raila Odinga by 54.8% to 44.3%.
But Mr Odinga, a former political prisoner who unsuccessfully used the courts to contest his defeat in the last presidential election in 2013, dismissed the count as “fictitious” – with the opposition’s own tally indicating he is winning.
The final days of campaigning in Kenya’s election were marred by the murder and torture of a top election official, which Mr Odinga linked to his claims of vote rigging.
At a late-night news conference, he added: “We have our projections from our agents which show we are ahead by far. We fear this was exactly the reason Chris Msando was assassinated.”
Kenya’s election has been dogged by fears of another outbreak of violence similar to the one seen after Mr Odinga’s election loss in 2007.
That crisis, fuelled by ethnic divisions, led to the deaths of about 1,200 people.