STORY: Ruto called on voters to vote carefully and said he was, 'confident that the people of Kenya will make the right choices.''
Ruto and veteran opposition leader Raila Odinga are leading in the race to succeed President Uhuru Kenyatta, who is stepping down after serving since April 2013, due to constitutional term limits.
Ruto has been President Uhuru Kenyatta's deputy for the past nine years, though the two men have fallen out.
Ruto has sought to capitalise on growing anger among poor Kenyans and says he plans to create a fund to provide loans for small enterprises.
Voters started to cast their ballots shortly after the official opening of the polls at 6 a.m. local time (0300 GMT).
There are 22.1 million registered voters. Provisional results will start streaming in on Tuesday night, but an official announcement will take days.