A bomb has killed at least two people, including a police officer, and injured at least 10 more people at a rally in Ukraine’s second city Kharkiv.
The rally was one of several being held to mark a year since the Kiev uprising that led to the fall of pro-Russia leader Viktor Yanukovych.
Security forces have detained four suspects in the attack, officials say.
Kharkiv lies outside the conflict zone in eastern Ukraine, where a ceasefire appears finally to be taking hold.
The government has agreed to start pulling back heavy weapons from Sunday, and the rebels said they would start the process on Tuesday.
The pullback will not be completed until at least 8 March, five days later than the deadline set at peace talks in Minsk this month.
Another key element of the Minsk deal moved forward on Saturday when the Ukrainian government and the rebels exchanged 191 prisoners.
The BBC’s Paul Adams in Donetsk says these two factors provide a glimmer of hope for the deal, but with so much suspicion and bad faith between the two sides, no-one should assume immediate results.
The ceasefire continues to be breached, notably in Debaltseve, a key transport hub captured by the rebels in recent days, an OSCE official said.
Alexander Hug added that the humanitarian situation there was “relatively catastrophic”.
“The local population reported to us that there is no water, no food, no gas, no heating, no electricity, no medication. And all the buildings that our monitors have seen (…) have been affected by the fighting,” he said.