At least two people have died after a bomb exploded at a rally in Kharkiv to mark a year since the toppling of pro-Russia leader Viktor Yanukovych.
Eyewitness Alexei Grechnev told the BBC: "We were walking in a column of people, in the front section, when I heard a loud boom and saw some people fall to the ground a few metres away from me, a somewhat unexpected and unreal scene, like in movies."
Ukraine's second largest city is near the Russian border but is outside the conflict zone.
The ceasefire, in effect since last week, continues to be breached by both sides, but the government has agreed to withdraw heavy weapons from Sunday, with rebels agreeing to do the same from Tuesday.
Dozens of prisoners were exchanged between pro-separatist rebels and the Ukrainian military on Saturday, the first step of the February 12 ceasefire agreement which was successfully carried out.
US considering 'serious sanctions' against Russia (21 February 2015)
France, Germany and Ukraine call for free access to ceasefire observers (16 Feb 2015)
Ceasefire comes into effect in Ukraine (15 Feb 2015)