Vitali Klitschko says it is a "mistake" for Russia to help former president Victor Yanukovich.
Asked about a press conference the former president is due to hold in Russia later he told ITV News:
"I think it's a mistake for the Russian Federation to help a man like that. A man who shot his people and held all the power.
"Yanukovich has to take responsibility for what has happened in this country. We have a huge political and economic crisis as a result of four years of his rule. He promised to listen to the people and he did not. It's all a result of his total corruption in Ukraine.
"People don't have justice and right now everyone is very upset. People want change. Everyone hopes we can change our country.
"We see right now the result of his power. I told him personally to step back months ago. If he had done that we wouldn't have the huge political crisis we have now in Ukraine."
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.