Kenyans are choosing their next president after an intense campaign dominated by debates about living costs, unemployment and corruption.
Outgoing President Uhuru Kenyatta, having served the constitutionally limited two terms, is backing one-time foe Raila Odinga, 77, to succeed him.
His decision followed a falling out with Deputy President William Ruto, 55, who had expected to be endorsed.
More than 22 million Kenyans have registered to vote.
There are several other elections happening at the same time and a mix-up of ballot papers in some areas for some of those votes has again raised questions about the organisation of the general election.
Polls are open for 11 hours from 06:00 local time (03:00 GMT). Anyone still in the queue at closing time will be allowed to vote.
The results of the last presidential election in 2017 were annulled after the Supreme Court ruled that the electoral commission had not followed the law when it came to the electronic transmission of the vote tallies from the polling stations.
Judges ruled that “illegalities and irregularities” had taken place.
A re-run was won by Mr Kenyatta, but boycotted by Mr Odinga – the main opposition candidate at the time.
The chairman of the electoral commission, Wafula Chebukati, who was also in charge of the 2017 vote, has frequently tried to reassure Kenyans that his team will be up to the task this time.
Counting will start at the polling stations shortly after voting ends. Officials will then take a photo of the final tally and send the image to both the constituency and national tallying centres.
To ensure transparency the media, political parties and civil society groups have been urged to run their own tallies using final results declared at the more than 40,000 polling stations.
But only the electoral commission can declare the winner of the presidential election after verifying the physical and digital forms sent to the national tallying centre.
The main presidential candidates have vowed to respect the result of the elections.
Source: BBC