Vitali Klitschko has condemned the Russian government for their involvement in Crimea and for hosting Ukraine's ousted president Victor Yanukovich.
Mr Klitschko, who will stand in the presidential elections in May, told ITV News this morning:
"The situation in Crimea is very critical. It is difficult to understand why Russia has sent a Government representative to Crimea but we are doing everything we can to keep the situation under control.
"There is a risk the situation could spread and dealing with that is the main task for politicians in Parliament today and also should be for politicians around Europe. We have to let the politicians try and sort this out not the police or military people."
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.