Reaction to the result could partly depend on the performance of Kenya's electoral commission, which will collect vote counts from more than 40 000 polling stations.
The final week of campaigning has been marred by the murder of a top election official and claims of vote-rigging.
United Nations human rights experts weighed in on election eve, urging Kenyans to ensure a peaceful vote and avoid repeating the violence of 10 years ago.
Millions of registered voters in Kenya go to polls today in one of the most closely watched elections on the African continent.
The President called for peace, urging Kenyans to vote peacefully and go back to their homes to await the results.
Obama said he was disappointed with the campaign so far, which has been marked with a raid on the vice president's country estate, and reports of a rigged vote in favor of Kenyatta. Since Uhuru Kenyatta became president in 2013, however, the country has struggled to combat deadly terrorist attacks and has been plagued by corruption scandals.More news: U.S. to Exempt Journalists, Aid Workers from North Korea Travel Ban
Odinga, 72, has accused Kenyatta of attempting to rig this year's election.
The polls are seen as a litmus test of Kenya's progress since a disputed 2007 election sparked two months of violence which left more than 1,100 people dead and 600,000 displaced.
What happens to Kenya is less about who wins the elections and more about how those who lose take their defeat.
Kenyatta, the son of Kenya's founding president and the country's youngest leader at 55, is seeking a second five-year term.
Nervous Kenyans stockpiled food and water yesterday and police prepared emergency first aid kits as families headed to their ethnic heartlands on the eve of an election many fear could descend into violence.
"I have been voting in this place but I have never seen such a crowd turnout ever since; the turnout is very huge; people wanted to exercise their right to vote", he said.More news: 19 countries officially apply to host the EMA; 8 chase EBA
Kenya will be conducting general elections Tuesday for the presidency and more than 1,800 elected officials, including governors, legislative representatives and county officials. The IEBC extended voting hours in the affected areas and deployed helicopters to ferry materials and polling officials to the stations.
In Kenyatta's home town Gatundu, north of Nairobi, a voter who identified herself only as Gathoni also arrived early to "vote for my favourite president", who will himself cast his ballot at the polling station later in the morning.
Vote counting got underway in several parts of Kenya Tuesday evening following close of polling at the official time: 5pm EAT.
Despite the lack of attention, the election is evidence of the larger role women are playing in politics in Kenya, where only 19% of parliament seats are held by women, the lowest proportion in East Africa.
Many polling stations closed on schedule at 5 p.m. local time (1400GMT), though some sites remained open to process voters still waiting to cast ballots.
Former US Secretary of State John Kerry is among thousands of observers who are monitoring the election.More news: Eastern Missouri officer shot and killed during traffic stop