The world's leaders seem to have been caught thoroughly off guard by the speed with which this has escalated.
Foreign Secretary William Hague has called in the Russian Ambassador to ask for an explanation, he's also asked for and got a meeting of the UN Security Council tonight. But Russia wields a veto on that body and it's not at all clear what they can do about it.
President Obama's warnings last night that action against Ukraine would have a costs seem to be being ignored, but what we don't know is what President Putin's intentions are.
Does he really intend a shooting war with Ukraine? Is he gambling that Ukrainians simply won't fight? Or is he simply raising the tension so high now that when he backs down and doesn't invade the world breathes a sigh of relief and accepts his conquest of Crimea.
Any one of these is possible.
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.