At least two people have been killed after a blast struck a crowd at a pro-Ukrainian rally far from eastern Ukraine's war zone, and Kiev said it had arrested suspects who had been armed and instructed in Russia.
Ukrainian authorities fear violence could spread from two rebellious provinces to other areas in the mainly Russian-speaking east, a swathe of territory which the Kremlin calls "New Russia".
The bomb in Kharkiv, the biggest city in the east, struck one of numerous events across Ukraine on Sunday marking the deaths of 100 protesters a year ago in an uprising that toppled a Moscow-backed president.
"Today is memorial Sunday, but on this day terrorist scum revealed its predatory nature," President Petro Poroshenko said on Facebook. "This is a brazen attempt to expand the territory of terrorism."
Amateur footage posted on the Internet showed a few hundred marchers with Ukrainian flags shouting "glory to the heroes!" when the blast occurred. Demonstrators and bystanders, including a woman pushing a baby in a pram, fled in panic. A wounded man in military uniform lay in snow crying for help.
