A bomb has killed at least two people, including a police officer, and injured 10 other people at a rally in Ukraine’s second city Kharkiv.
According to BBC, 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 several 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 and rebels on the frontline have agreed to start pulling back heavy weapons from Tuesday.
The process 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 blast took place during a march to mark the one-year anniversary of clashes in the capital Kiev, which left more than 100 people dead and drove the then president, Viktor Yanukovych, from power. Petro Poroshenko, the present president, and a number of European leaders attended the main march in Kiev.
The explosion follows other blasts in Kharkiv and elsewhere in Ukraine. Kharkiv lies outside the conflict zone in eastern Ukraine but there have been concerns that Russian-backed separatists could target it as well.
In Kiev, the main march went peacefully. President Poroshenko and European leaders led a procession of thousands around the main locations where protesters fought riot police one year ago.
The explosion in Kharkiv happened at 13:20 (11:20 GMT) as people gathered near the city’s Palace of Sport for a march in support of national unity, Ukrainian media say.
An unnamed regional prosecutor quoted by Reuters news agency said an explosive device had apparently been thrown from a car.
A photo apparently taken at the scene shows a body draped in a Ukrainian flag as ambulances stand nearby.
“Ukraine’s security service detained persons who may have been involved in the preparation and carrying out of crimes of a terrorist nature in Kharkiv, including the explosion,” security service spokesman Markian Lubkivskyi wrote on his Facebook page.