An international election observer says Kenyans are voting with enthusiasm and that the process is going well.
John Mahama, chief election observer for the Commonwealth and former president of Ghana, said early on Tuesday that he is impressed by the voter turnout.
"There have been no incidents so far," Mahama said. "Voting seems to be going smoothly and I think it is a good sign for Kenyan democracy."
Kenya has 20 million registered voters out of a population of more than 40 million people. They are voting in a fiercely contested election that pits President Uhuru Kenyatta against challenger Raila Odinga.
A 102-year-old woman believed to be one of Kenya's oldest citizens said she has voted for incumbent president Uhuru Kenyatta.
Lydia Gathoni voted in Kenyatta's home town, Gatundu, north of Nairobi.
"I have come here to vote because good leadership comes from God. I want to vote because I believe God has kept me alive for so many years," said Gathoni.
Kenyans on Tuesday started voting in an election that pits Kenyatta against challenger Raila Odinga in this East African economic hub known for its relative, long-term stability as well as the ethnic allegiances that shadow its democracy
Election officials say the winner of Kenya's presidential race must get more than 50 percent of the votes as well as one-quarter or more votes in at least 24 of Kenya's 47 counties. If the front-runner falls short of those benchmarks, the two top contenders will contest a run-off vote.
President Uhuru Kenyatta and challenger Raila Odinga also faced off in the 2013 election. Kenyatta won by a thin margin, with just over 50 percent of the vote; Odinga alleged vote-tampering and took his case to Kenya's highest court, which ruled in Kenyatta's favor by validating the results.
Like many Kenyan voters, 34-year-old Fred Nyakundi arrived at a polling station several hours before it opened and waited in line in the dark before casting his vote.
"The exercise is very slow, but I am happy with the service I got," said Nyakundi, who owns a carpentry business in Nairobi. "I am going home to open the business and wait for results."
Another voter, 41-year-old Fatuma Ramadhan, thought the voting procedure was speedy. She was able to vote at 6 a.m., when polling stations opened, and then opened her restaurant to serve breakfast to other voters.
Early voters went to the polls across the country at sunrise. Election officials say the polls will close at 17:00.
Vote counting will start immediately and results are expected within 24 to 48 hours, although authorities have up to a week to deliver official, final results.
Kenyans are voting in an election that pits President Uhuru Kenyatta against challenger Raila Odinga in an East African economic hub known as much for its relative, long-term stability as the ethnic allegiances that shadow its democracy.
A key concern after polls opened Tuesday was whether Kenya would echo its 2013 election, a mostly peaceful affair despite opposition allegations of vote-tampering, or the 2007 election, which led to violence fueled by ethnic divisions that killed more than 1 000 people.
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. Kenya has nearly 20 million registered voters.
Former US Secretary of State John Kerry is among thousands of observers who are monitoring the election.