Voting got underway shortly after 6am local time and all eyes are on an electronic voter identification and tallying system whose success is seen as crucial to a smooth election.
Last week, one senior election official was found murdered, his body displaying signs of torture.
"We shall extend the time of voting for the same amount of hours lost before opening", said IEBC chief Wafula Chebukati.
Their votes will matter in some of the most contested parts of the country.
Millions of registered voters in Kenya go to polls today in one of the most closely watched elections on the African continent.
Almost 20 million Kenyan voters were scheduled to elect a new president for East Africa's richest economy in a tight race on Tuesday, which pitted incumbent President Uhuru Kenyatta against his longtime rival, Raila Odinga.
"We must not, and I repeat, we must not disappoint them".
Were Raila to win, it would upend the political dominance of the Kikuyu ethnic group, which has supplied three of Kenya's four presidents since independence from Britain in 1963.
Editar Ochieng, a resident of the Nairobi slum Kibera, where clashes broke out in 2007, has sent her two daughters out of town for the duration of the elections.
In 2007, more than 1,100 Kenyans died and 600,000 were displaced after a disputed election - an outcome neither side wants to see repeated.
Saira Banu hopes for speedy recovery of Dilip Kumar
In April past year , he was hospitalised due to fever and nausea. "He will need a dialysis", Parkar was quoted as saying. Dilip Kumar, whose real name is Yusuf, is sitting and appearing stable albeit weaker than usual.
What happens to Kenya is less about who wins the elections and more about how those who lose take their defeat.
Odinga is the son of the country's first vice president and Kenyatta is the son of its first president.
The contenders have called for a peaceful vote amid widespread fears that the crucial election could be marred by communal violence. Kenyatta defeated Odinga in a largely peaceful poll in 2013, and he is now running for a second term.
Odinga comes from the Luo people in western Kenya, an area that has long felt neglected by the government and resentful of their perceived exclusion from political power.
Kenyans are voting to elect the president, members of the senate, county governors, county women representatives, members of county assemblies and members of parliament in 40,883 polling stations across the country. The disabled and the old have also been moved to voting centres around the country to cast their votes.
Officials and politicians have called for calm in what is mainly a contest between sitting president Uhuru Kenyatta 55 and former Prime Minister Raila Odinga 72.
The Latest on Kenya's elections to be held on Tuesday.
More than 180,000 agents from various organs of state security are also believed to have been deployed for the election.
Odinga, who leads the National Super Alliance (NASA), warned of possible rigging and intimidation on the eve of the elections while addressing thousands of people.
"Everyone is coming out early and in big numbers because we need change so badly", said Anne-Marie Omondi, a 28-year-old Odinga supporter in Kisumu, an Odinga stronghold in western Kenya swept up in the 2007 unrest.