Kenyans are going to the polls to elect their president amid concerns the vote could spark ethnic violence.
Long queues formed outside many polling stations before dawn as voters waited to choose between President Uhuru Kenyatta and challenger Raila Odinga.
The 2007 election led to violence fuelled by ethnic divisions that killed more than 1,000 people, and while 2013's vote was mostly peacefully - despite allegations of vote-tampering - there are concerns about what the fallout from the result might be.
The reaction could partly depend on the performance of Kenya's electoral commission, which will collect vote counts from more than 40,000 polling stations.
Fears of violence were increased by the murder of an electoral official in charge of technology days ahead of the election.
The election commission has said that about 25% of polling stations will not have network coverage, meaning officials will have to move to find a better signal and transmit results by satellite phones.
Kenya's nearly 20 million registered voters will cast ballots in the tightly-contested race for the presidency as well as for more than 1,800 elected positions, including governors, legislative representatives and county officials.
Many people are expected to vote along ethnic lines.
President Kenyatta is widely seen as the candidate of the Kikuyu people, the country's largest ethnic group, while Mr Odinga is associated with the ethnic Luo voting bloc, which has never produced a head of state.
In a speech on Monday night Mr Kenyatta appealed to Kenyans to vote peacefully in large numbers.
"How you have voted should not in any way reflect or change the manner in which you have related to your neighbour," he said.
"Shake their hand, share a meal and tell them, 'Let us wait for the results', for Kenya will be here long after this general election."