Hundreds of pro-Russian militants swinging batons and wearing helmets on Friday attacked a rally of 1,500 people in Ukraine's southern port city of Odessa demonstrating for national unity, an AFP journalist said.
Police intervened to try to break up the violence, which left dozens wounded on both sides.
Odessa, which has around one million inhabitants, had up to now been relatively spared the unrest wracking east Ukraine.
However, the city is located close to the border with Moldova and its breakaway state of Transdniestr, where Russia has troops stationed since a short war in 1992.
Ukraine's Western-backed government accuses Russia of fomenting the rebellion on its territory.
On Monday, dozens of pro-Russian thugs armed with bats, bricks and knives attacked another pro-Ukrainian rally in the eastern city of Donetsk, wounding 14.