‘I have complete confidence in the residents of Kiev residents and am confident that the principles we are upholding correspond to their interests,’ Klitschko said after casting his ballot.
Klitschko, known as ‘Dr. Iron Fist’ because of his doctorate in sports and one of the few champions never to have been knocked down in his career, voted with his brother Vladimir, the current IBF and WBO boxing champion.
The 36-year-old has earned popularity with his image as a corruption fighter and has used his international celebrity status to boost his profile but current mayor Leonid Chernovetsky remains ahead of him in the polls.
Polling stations opened at 7:00 am (0400 GMT) and were to close at 10:00 pm (1900 GMT). The results of exit polls were expected soon after with the first official results early on Monday.
Seventy candidates were running for the mayoral election.
‘I have always achieved my goals. Now I'm fighting for my city. I want to turn it into an international metropolis,’ Klitschko said during a visit to New York where he recruited the support of former mayor Rudolph Giuliani.
In March, the Ukrainian parliament ordered early elections following corruption accusations against Chernovetsky, a former banker.
Chernovetsky, who defeated Klitschko in the 2006 mayoral vote, remains popular however, largely thanks to City Hall subsidies for the poor.
This time, Klitschko's main campaign promises have been to end the chaotic construction practices that have erased many of the city's green spaces, reduce traffic, fight corruption and improve social welfare.
‘Kiev used to be a museum city, a park city. Now it is being transformed into a cement jungle,’ Klitschko, a member of the city assembly, said in an interview last year with AFP.
Klitschko was born on July 19, 1971 in Kyrgyzstan, a Central Asian state that was then part of the Soviet Union, into a Soviet officer's family. They then moved to Ukraine.
He took his first steps on the Ukrainian political scene during the pro-democracy Orange Revolution of 2004, backing pro-Western opposition leader Viktor Yushchenko, who went on to become president.