The polls opened at 06:00 (03:00 GMT) with tens of thousands of police and other security staff deployed to protect voters and polling stations.
International observers have scaled down their missions for security reasons.
One voter in Nairobi's Mathare slum, taxi driver David Njeru, 26, told the AFP news agency: "It is my duty to vote. Last time the queue was all around the block and I waited six hours to vote, this time the people are few."
After casting his vote in the town of Gatundu, Mr Kenyatta urged people to cast their ballots so the country could move on.
"We're tired as a country of electioneering. It's time we moved forward," he said, adding that most of the country was "calm and peaceful".
Ethnic-based politics is deep-seated in Kenya, fuelled by the illusion that if "one of us" is in power then people from that community will benefit, he says.
Some Kenyans may also feel Mr Odinga's withdrawal makes the competition somewhat redundant; others may be just fed up with the ongoing political crisis - all factors that could affect the overall turnout, he adds.
Mr Odinga had vowed to disrupt Thursday's vote, calling for "massive" demonstrations, but has asked his supporters to stay away from polling stations in an attempt to avoid violent clashes.
In a statement earlier this month, he accused the government of instituting a "dictatorship", adding: "We are going to win the battle for a free and fair election".
"Our opponents want an election for the sake of it, we want a better election," he said.
Crossposted from: http://www.bbc.com/news/world-africa-41757612