Pro-Russia activists recaptured the administrative headquarters of the eastern Ukrainian city of Donetsk and flew the Russian flag from its roof, just hours after Ukrainian authorities managed to take back the building.
However, there remains pro-Kiev supporters in the region as around 5,000 activists gathered to chant "Down with Putin! Donetsk is Ukraine!" in the city's central square.
However, just hours later around 200 protesters re-entered the building, despite a huge police presence.
Demonstrators ripped metal grills from reception, and hung out of windows claiming victory before raising the Russian flag.
Self-appointed leader of the movement Pavel Gubarev said he wanted the east of Ukraine to sever ties with Kiev, and said he would be willing to have Russian troops in Donetsk.
Donetsk has been the centre of pro-Russian demonstrations since Saturday, as tension between Ukraine and Russia escalated.
Kiev has claimed that the protests were arranged by Russia to justify an invasion.
David Cameron and Barack Obama said the EU and US should "stand united in their condemnation of Russia's actions" after a phone conversation.
Nato announced a review into its cooperation with Russia due to the ongoing crisis, and the Pentagon said it was sending six additional F-15 fighter jets to assist the Baltic air policing operation.