Vitali Klitschko told ITV News the only way to unite Ukraine was to solve the economic problems and tackle corruption.
Every Ukrainian, it doesn't matter in which part, south or north, east or west they are looking for life standards, to have a job, to have justice.
We need to reform things so we get a government who can change the standards of life for people. That idea can unite a whole country.
Pro-Russia activists recaptured administrative headquarters in the Ukrainian city of Donetsk and flew the Russian flag from its roof.
ITV News' Europe Correspondent describes the tense atmosphere surrounds a central government building in Donetsk .
New footage obtained by ITV News shows Russian troops firing warning shots as Ukrainian soldiers approached during a tense stand-off today.