A bombing at a pro-government demonstration in which at least two people were killed has marred advances on a peace plan for eastern Ukraine.
At least nine people were wounded by the nail-packed bomb in the eastern city of Kharkiv during a memorial event for Ukrainian soldiers killed fighting pro-Russian rebels.
The Interior Ministry called it a terrorist attack and dispatched special forces to Kharkiv.
Four people accused of taking part in the bombing on Sunday were arrested, media reports said, citing the Security Council.
Weapons, including a grenade launcher, were confiscated during the operation, it said.
There have been several bombings in Kharkiv and other majority-Russian-speaking cities, but they usually only result in damage to property.
The bombing followed successes in implementing a peace plan brokered this month by Germany and France.
The Ukrainian government and pro-Russian rebels agreed in writing to begin withdrawing heavy weapons on Sunday from the conflict zone in the country's east, both sides said.
Government spokesman Pyotr Kanonik said the removal would be completed within two weeks, local media reported.
"The document was signed during the night," separatist spokesman Eduard Bassurin said in the rebel-held city of Donetsk.
"Everyone has agreed to and confirmed the two-week period from February 22."
A co-ordinated arms removal was stipulated in the ceasefire accords signed this month in Minsk, but the implementation has been delayed because of fighting in the rail hub of Debaltseve, which government troops pulled out of on Wednesday.
On Saturday, the two sides also swapped 191 prisoners.
Meanwhile, in Kiev, a "March of Dignity" to mark the one-year anniversary of the Maidan revolution was held without any violent incidents.
More than 100 pro-Western demonstrators were killed a year ago when then-president Viktor Yanukovych cracked down on those calling for his ouster.
He fled to Russia on February 22, 2014.
Petro Poroshenko, who was subsequently elected to replace him, invited the presidents of Germany, Poland, Lithuania, Slovakia and Georgia to attend the march on the Maidan, Kiev's central square, where the pro-democracy and pro-EU protests were centred.
Poroshenko walked arm in arm with German President Joachim Gauck and other foreign dignitaries past memorials to the victims and on towards the Maidan, which means Independence Square in Ukrainian.
About 10,000 people were estimated to have turned out for the demonstration.
Many waved the Ukrainian flag and carried signs that read, "We are Europe".
Gauck said he visited Kiev to show "solidarity with the Ukrainian democracy movement".
The march came a day after about 40,000 anti-Maidan protesters turned out in Moscow, criticising the EU and NATO stance on Ukraine.
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