Clashes broke out in the flashpoint eastern Ukrainian city of Donetsk on Wednesday following rival demonstrations by thousands of pro- and anti-Russian protesters, leaving around 10 people injured, AFP reports.
There were fistfights and militants hit each other with sticks in the city centre, an AFP reporter witnessed. Several people were also seen being arrested and protesters with bloodied faces were carried away.
Pro-Russian protesters also took back the regional government building a few hours after being expelled from it, pushing through ranks of riot police with cries of "Russia!" and "Fascism will not pass!"
Pro-Russian forces had already taken over the building on Monday and hoisted the Russian flag in a city that is a stronghold of Ukraine's former pro-Moscow president Viktor Yanukovych, ousted last month.
They elected a new "governor" for the region, Pavel Gubarev, refusing the authority of the one named by the new pro-European authorities in Kiev, Sergiy Taruta.
The Ukrainian prosecutor's office on Wednesday said it was opening an investigation against Gubarev for "threatening the territorial integrity" of Ukraine.
Pro-Russian militants in the regional government were evacuated by police earlier Wednesday under the pretext of a bomb threat and the Russian flag was lowered to be replaced again by the Ukrainian blue-and-yellow.
"We will not give our region to Taruta, no one will support him," one man, who declined to be named, told AFP at a protest of around 2,000 pro-Russian activists.
At the rival demonstration for the territorial integrity of Ukraine, around 10,000 people took part.
There have been several demonstrations against the new authorities in Ukraine in eastern and predominantly Russian-speaking regions of the former Soviet republic.