The election is considered close, and East Africa's economic hub could see a presidential runoff for the first time. Economic issues such as widespread corruption could be of greater importance than the ethnic tensions that have marked past votes with sometimes deadly results.
Kenya is a standout with its relatively democratic system in a region where some leaders are notorious for clinging to power for decades. Its stability is crucial for foreign investors, the most humble of street vendors and troubled neighbours like Ethiopia and Somalia.
Hundreds of voters lined up on Tuesday hours ahead of polls opening in some locations, often after being summoned by volunteers' early morning whistles. Voting started late in some areas as materials or polling workers were delayed.
The top candidates are Raila Odinga, a democracy campaigner who has vied for the presidency for a quarter-century, and 55-year-old Deputy President William Ruto, who has stressed his journey from a humble childhood to appeal to millions of struggling Kenyans long accustomed to political dynasties.
"In moments like this is when the mighty and the powerful come to the realisation that it is the simple and the ordinary that eventually make the choice," a smiling Ruto told journalists after becoming one of the first voters.
"I look forward to our victorious day." He urged Kenyans to be peaceful and respect others' choices.
"I have confidence that the people of Kenya are going to speak loudly in favour of democratic change," Odinga told journalists on his way to vote.
Outgoing President Uhuru Kenyatta, the son of Kenya's first president, cut across the usual ethnic lines and angered Ruto by backing longtime rival Odinga after their bitter 2017 election contest. But both Odinga and Ruto have chosen running mates from the country's largest ethnic group, the Kikuyu.
The 77-year-old Odinga has made history by choosing running mate Martha Karua, a former justice minister and the first woman to be a leading contender for the deputy presidency.
"Make your voice heard," she said after voting early in a knitted cap, a sign of the unusually cold weather in parts of the country.
Rising food and fuel prices, huge government debt, high unemployment and corruption mean economic issues are at the centre of an election in which unregulated campaign spending highlighted the country's inequality. But personalities still matter.
Kenyans are hoping for a peaceful vote. Elections can be exceptionally troubled, as in 2007 when the country exploded after Odinga claimed the vote had been stolen from him and more than 1,000 people were killed.
Ruto was indicted by the International Criminal Court for crimes against humanity for his role in violence, but his case was terminated amid allegations of witness tampering.
Ruto and Odinga have said they will accept the official results — if the vote is free and fair.
"It is every Kenyan's hope," the president told journalists after voting.
More than 22 million people are registered to vote. Official results must be announced within a week, but impatience is expected if they don't come before this weekend. The underfunded Independent Electoral and Boundaries Commission is under pressure to ensure an untroubled vote.
To win outright, a candidate needs more than half of all votes and at least 25 per cent of the votes in more than half of Kenya's 47 counties. No outright winner means a runoff election within 30 days.
