KIEV, Ukraine - A bomb blast killed two at a march in the east on Sunday on the first anniversary of the ouster of the country's Russia-friendly president, a gruesome counterpoint to the hopes spawned by a cease-fire agreement in Ukraine's conflict with separatists more than a week ago.

An exchange of prisoners this weekend and acknowledgment by both sides of a commitment to pull back heavy weapons are promising signs that peace may yet prevail, although sporadic exchanges of hostilities between government and separatist forces have yet to subside entirely.

Ukraine's Interior Ministry said the blast at a march in the eastern city of Kharkiv to mark the ouster of President Viktor Yanukovych was due to an "unknown explosive device" and was considered a terrorist act. A police officer was one of the dead and about a dozen people were injured, the ministry said.

A spokesman for the national security service, Markian Lubkivskiy, said four suspects had arrested for the explosion and for planning other attacks, Interfax-Ukraine news agency reported.

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The violence comes as Ukraine continues to be riven by tension and bloodshed stemming from Yanukovych's fall. The Ukrainian parliament voted Feb. 22, 2014 to remove him, following months of increasingly violent protests in the capital, Kiev.

The Crimean peninsula, where residents largely regarded his downfall as a coup, was annexed by Russia a month later.

Then armed rebels opposed to the new authorities in Kiev took over large parts of two regions bordering Russia, setting off a war that has killed more than 5,600 people.

A peace plan envisioning a cease-fire and pullback of heavy weapons was signed Feb. 12.

Ukraine planned to begin withdrawing heavy weaponry from the front lines on Sunday, Ukrainian military spokesman Col. Andriy Lysenko told a briefing, but gave no details.

Rebel spokesman Eduard Basurin said the pullback from both sides is to take place between Sunday and March 7, but he did not say whether rebels had made any moves yet. There was no immediate confirmation that the withdrawal had begun.

Both sides are to pull back their big guns and rockets from 25 to 70 kilometers (15 to 43 miles) away from the conflict line - depending on the weapons' size - creating a buffer zone of between 50 and 140 kilometers (31-87 miles).

The buffer zone was a main element of a peace agreement worked out by leaders of Russia, Ukraine, Germany and France. Late Saturday, 139 Ukrainian soldiers and 52 rebels were exchanged, as called for in the peace plan; it remains unclear how many prisoners may remain on each side and when any other swaps might take place.

The cease-fire that was the first element of the Minsk plan was called into effect last Sunday.

Ukraine said Russia-backed separatists violated the cease-fire a dozen times during the night with artillery and rocket attacks and an attempt to storm a Ukrainian encampment. Lysenko said one serviceman was killed and three wounded over the past day.

Explosions were heard in the main rebel-held city Donetsk around dawn on Sunday and a rebel website says several buildings were damaged by artillery.

Later in the day, sustained bursts of machine gun fire were heard coming from a government-held suburb adjacent to the city's now-obliterated international air terminal.

However, the level of firing appeared to be far lower than a week ago.

Among the attacks reported by the Ukrainian military was an attempt to storm positions in the village of Shyrokyne near the port city of Mariupol. Rebel seizure of Mariupol could help establish a land corridor between mainland Russia and the Crimean peninsula.

Associated Press journalists in Bezimenne, the final rebel-held village along the shore before Shyrokyne, heard sporadic outgoing blasts late Sunday afternoon.

"They fire at us, and we fire at them in return," said Oleg, a rebel fighter who said he came from the Russian city of Yekaterinburg. He declined to provide his surname as he has not told all his family he is fighting in Ukraine.

Tensions also remain high around Debaltseve, a strategic railway hub captured by separatists after intense battles last week. Rebel forces in Lohvynove, a village just north of Debaltseve, told AP reporters that they continue to be targeted by Ukrainian artillery.

"At the moment, it's quiet. But in a couple of hours, they will start to fire at our positions," said rebel fighter Alexander, who also identified himself by the nom de guerre Tok. As he spoke, an explosion in the distance prompted him to turn around sharply: "There it bangs again. Maybe something will fly this way."

In Kiev, thousands participated in a march on Sunday commemorating the events of a year ago and honoring the more than 100 protesters who died during the uprising. President Petro Poroshenko led the ceremony, joined by representatives including the presidents of Poland, Lithuania, Germany, Georgia and the European Union.