More than 22 million people, about 40 percent of them under 35, are registered to vote in an election held against a backdrop of soaring inflation, a punishing drought and disenchantment with the political elite.
Deputy president and erstwhile heir-apparent William Ruto, 55, is running against Raila Odinga, the 77-year-old veteran opposition leader now backed by longtime rival Kenyatta after a stunning shift in allegiances.
As neither Ruto nor Odinga belong to the dominant Kikuyu tribe, which has produced three of the country's four presidents, the election will open a new chapter in Kenya's history.
Lawyers David Mwaure and George Wajackoyah -- an eccentric former spy who wants to legalise marijuana -- are also standing for president but are likely to trail far behind the frontrunners.
Ruto was among the first to vote in his Rift Valley stronghold on what he described as "D-day". If Odinga wins, his running mate Martha Karua would become deputy president, the first woman to hold the post.
Ruto has painted the election as a battle between ordinary "hustlers" and "dynasties" -- the Kenyatta and Odinga families that have dominated Kenyan politics since independence from Britain in 1963.
Odinga, who is known as "baba" or father and is making his fifth stab at the presidency, later cast his ballot in the Nairobi slum of Kibera.
In his bastion in the lakeside city of Kisumu, the atmosphere was festive, with motorcyclists honking and blowing whistles.
Analysts have in recent days suggested that Odinga, a onetime political prisoner and former prime minister could edge past his younger rival.
If neither wins more than 50 percent, Kenya would have to hold a run-off for the first time in its history.
After a vitriolic campaign, voting was generally smooth.
But some incidents of delays in opening polling stations and problems with electronic voter registration equipment were reported.
And in one area in Nakuru county in western Kenya, police fired tear gas after youths blocked a road with burning tires.
The country is hoping for a peaceful transition of power after almost a decade under President Uhuru Kenyatta, but concerns about vote-rigging linger after past election disputes spiraled into bloodshed.
Both Odinga and Ruto have urged a peaceful election, but fears remain that if the loser challenges the outcome -- as widely expected -- there could be unrest.
No presidential election outcome has gone uncontested since 2002, and there will be an anxious wait for this year's results which are not expected for several days.
The trauma of the 2007 poll, which was followed by politically motivated ethnic clashes that killed more than 1,100 people, runs deep.
Odinga's challenge to the 2017 election result that saw then foe Kenyatta re-elected was met with a heavy-handed police response that left dozens dead.
In a historic first, the Supreme Court annulled the 2017 vote, citing widespread irregularities.
Clara Otieno Opiyo, a 35-year-old vegetable seller who travelled before dawn to vote with her five-year-old boy strapped to her back, said she hoped her vote would ease economic pain for working-class Kenyans like herself.
"I came here at 4am to vote, having a lot of hope and faith, but if my presidential candidate succeeds, my children's schooling will be free, I will find work, and my life will change."
The new president will face challenges to tackle the cost-of-living crisis, youth unemployment, a $70-billion debt mountain and entrenched corruption.
Already battered by the Covid pandemic which threw hundreds of thousands out of work, Kenyans are now suffering from the Ukraine war, which has sent prices of basic goods spiraling.
"Our country is now full of corruption, we want someone to deal with the issue permanently," said first-time voter Ibrahim Ahmed Hussein, a 23-year-old student, in Kibera.
"I am voting so as to choose a leader who will change this country totally. Now we want to see a new change for the new generation."