President Uhuru Kenyatta is seeking a second and final term in office and is facing his long-time rival, Raila Odinga.
The election has already been subject to violence and claims of vote-rigging.
Opposition Leader Raila Odinga says a lack of high-speed mobile phone coverage in some areas means the electronic voting system being used could prove unreliable.
"We are saying that the law says Kenyans must cast their ballots through the electronic voting system which is called EVID, Electronic Voter Identification Devices, and we have said that if that system does not work, this vote count cannot go on. This is what we have said to our supporters everywhere, because what was going to happen was not valid. We are saying that if this happens, that polling station should be shut down until the changes are made. That is the law."
Kenya's electoral authority, the Independent Electoral and Boundaries Commission, says it will use satellite phones to transmit electoral results in areas without 3G or 4G mobile coverage.
The Commission's chairman is Wafula Chebukati.
"We wish to assure you all that we have taken steps to ensure that the results are electronically transmitted from all the polling stations through satellite transmission. We have provided satellite capability to all the 290 tallying centres from where presiding officers will take their results for transmission in any event."
A computerised voting system used in the 2013 election failed, forcing the manual counting of votes.
Afterwards Mr Odinga turned to the courts, alleging electoral fraud but lost his case.
In 2007, more than 1,100 Kenyans died and 600,000 were displaced after elections results were disputed - an outcome neither candidate wants repeated.
President Kenyatta has made a televised address urging Kenyans to respect the result and not resort to violence.
"No matter the result of this election, we must stand together as one people. Above all, we must reject intimidation. We must reject violence or any attempt to divide us."
The opposition also says the deployment of at least 150,000 members of the security forces is designed to intimidate voters, an accusation rejected by Kenya's Interior Minister, Fred Matiangi.
"Our security forces are not at the polling stations to hurt the people, they are there to facilitate our people, they are there to ensure the citizens of this nation enjoy, exercise and they are free to do so as their democratic right in terms of voting."
The lead-up to the election was marred by violence with an election official in charge of the computerised voting system, Chris Msando, tortured and strangled to death.
There are eight presidential candidates and if there's no outright winner, a run-off election will be held between the top two candidates.
