KHARKIV, Ukraine, Feb. 22 (UPI) -- A bomb exploded with deadly results in Ukraine's second-largest city on Sunday while thousands marched in rallies marking uprisings in Kiev one year ago.
At least two people, including a police officer, were killed in the blast and 10 people were injured.
The attack targeted hundreds who had gathered at the Palace of Sport in Kharkiv, which lies near the Russian border, in one of many rallies being held on the one-year anniversary of uprisings that led to the ouster of pro-Russian leader Viktor Yanukovych.
"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 meters away from me, a somewhat unexpected and unreal scene, like in movies," witness Alexei Grechnev told the BBC.
The attack is one of several that has struck Kharkiv in recent months, including a November explosion in a pub attended by pro-government activists.
Ukrainian officials say four suspects have been taken into custody and that they are Ukrainian citizens who received training and weapons in Belgorod, across the border in Russia. The four "may have been involved in the preparation and carrying out of crimes of a terrorist nature in Kharkiv, including the explosion," security spokesman Markian Lubkivskyi said, according to the BBC.
The attack comes a day after the exchange of nearly 200 prisoners between Ukraine and pro-Russian rebels -- the first step toward compliance with a Feb. 12 agreement brokered by France and Germany in Minsk. The agreement calls for a full prisoner exchange, a ceasefire and a pullback of heavy weaponry by both sides.
Despite pledges by the Kiev government to begin the pullback Sunday and for rebels to begin on Tuesday, the truce has not been followed in Debaltseve, which rebel forces seized last week after heavy fighting, claiming the ceasefire did not apply there.