From first-time voters to those bent with age, thousands descended upon polling stations, some from before midnight, to cast their ballots under heavy security.
Voting began relatively smoothly, with minor delays and technical hiccups, while all eyes are on a biometric voter identification and tallying system whose success is seen as crucial to a smooth election.
"I voted Raila, because he will be so much better to us. But if he does not win, it's ok. It's a democracy after all. Really, there's no need for violence," said Tom Mboya, 43, who works in construction and voted in the capital's largest slum Kibera.
Tensions soared in the last days of the campaign with the murder of a top election official in charge of the electronic voting system and opposition claims of a plot to rig the vote heightening a feverish atmosphere of conspiracy and suspicion.
The polls come a decade after a shambolic election -- which foreign observers agreed was riddled with irregularities -- sparked violence which left more than 1,100 dead and 600,000 displaced.
Odinga, 72, who is the flagbearer for the NASA coalition, is taking his fourth and likely final stab at the presidency.
He claims elections in 2007 and 2013 were stolen from him and right up until the eve of the vote, insisted that Kenyatta's Jubilee Party planned to rig Tuesday's presidential election.
Elections in 2013 were marred by the widespread failure of the electronic system, forcing officials to revert to manual counting of the vote. However Odinga took his grievances to the courts instead of the streets, where he lost.
The devolution of power to Kenya's 47 counties after a post-conflict constitutional reform means elections are now a complex affair, with citizens casting six different ballots.
Several tight races for posts such as governor have seen tensions flaring at the local level.
Nevertheless all eyes are on what is set to be the last showdown of a dynastic rivalry that has lasted more than half a century since the presidential candidates' fathers Jomo Kenyatta and Jaramogi Odinga went from allies in the struggle for independence to bitter rivals.
The men belong to two of the country's main ethnic groups, Kenyatta from the Kikuyu, the largest, and Odinga from the Luo.
Both have secured formidable alliances with other influential communities in a country where voting takes place largely along tribal lines.
Kenyatta, 55, is seeking re-election after a first term in which he oversaw a massive infrastructure drive and steady economic growth of more than 5%.
"He has done a lot for the country and he must absolutely be re-elected. He has built a lot of infrastructure, like the SGR train (between Nairobi and Mombasa), he has created jobs," said Evelyn Sum, 32, dressed in an elegant brown coat.
However Kenyatta is also criticised for soaring food prices -- with prices jumping 20% year-on-year in May -- and massive corruption scandals on his watch.
"Life is more and more expensive, especially the flour and the sugar. That's not good for poor people like us, and we hope that Odinga will change this," said Rose Lida, 48, wrapped in a red Maasai blanket on the chilly morning.
Obama urges peace
Former US president Barack Obama, whose father was born in Kenya, led a chorus of international calls on the eve of the vote for a peaceful election.
"I urge Kenyan leaders to reject violence and incitement; respect the will of the people," Obama said in a statement.
There are more than 19 million registered voters in the nation of 48 million. Half are aged under 35.
More than 150,000 security forces -- including wildlife, prison and forestry officers -- have been deployed for the vote, which ends at 5pm (1400 GMT).
Counting will begin immediately and the polls commission has a week to release final results.