At least two people have been killed in a bomb explosion at a pro-Ukrainian rally far from eastern Ukraine’s war zone. Kiev said it had arrested suspects who had been armed and instructed in Russia.

Ukrainian authorities fear violence could spread from two rebellious provinces to other areas in the mainly Russian-speaking east, a swathe of territory which the Kremlin calls “New Russia”.

The bomb in Kharkiv, the biggest city in the east, struck one of numerous events across Ukraine on Sunday marking the deaths of 100 protesters a year ago in an uprising that toppled a Moscow-backed president.

“Today is memorial Sunday, but on this day terrorist scum revealed its predatory nature,” President Petro Poroshenko said on Facebook.

“This is a brazen attempt to expand the territory of terrorism.“

Amateur footage posted on the internet showed a few hundred marchers with Ukrainian flags shouting “glory to the heroes!” when the blast occurred.

A Reuters journalist at the scene later filmed the bodies of two men lying by the road, draped in blue and yellow Ukrainian flags and surrounded by shrapnel.

Kharkiv is more than 200km from the war zone. It has occasionally seen violent protests by separatists in the past year but is now firmly under government control, and residents mostly support Kiev.

A Ukrainian prosecutor said one of those killed and four of the wounded were police who had been guarding the demonstration.

Demonstrator Igor Rossokha told Reuters Television his friend Igor was one of those killed: “We tried to give him first aid, but the paramedics arrived and said he‘d died instantly because he was struck in the heart.

“We just wanted to commemorate the anniversary, and this happened right at the start of our march,“ he said. “

The whole thing was called off immediately out of fear there could be more bombs on the route.“

Markian Lubkivskyi, an aide to the head of Ukraine’s SBU security service, said four suspects had been arrested. They were planning to conduct a series of attacks in the city with a Russian-made “Shmel“ rocket launcher.

“They are Ukrainian citizens, who underwent instruction and received weapons in the Russian Federation, in Belgorod,” he told Ukraine‘s 112 Television. Belgorod is a city across the nearby Russian border from Kharkiv.

Olexander Turchinov, secretary of Ukraine’s National Security and Defence Council, said Kharkiv was put under high alert in an anti-terrorist operation.

Moscow did not immediately respond to the accusations. It has long denied aiding its militant sympathisers in Ukraine.

Ukrainian officials initially said the blast was caused by an explosive thrown from a car, but later said a bomb had been buried under snow. The explosion follows blasts in other Ukrainian cities in recent weeks, although it appears to be the first in which fatalities were reported. Earlier, a senior pro-Russian rebel commander said separatist forces were due to begin withdrawing heavy weapons from the front line in east Ukraine, a sign rebels may be prepared to halt their advance as part of an internationally brokered peace deal.

Fighting has eased in many areas since a ceasefire came into effect a week ago, but the truce was shaken by the rebel capture on Wednesday of the town of Debaltseve. This forced a retreat by thousands of Ukrainian troops.

“The plan was signed last night ... Starting from today there are two weeks to withdraw heavy weapons,” Interfax news agency quoted rebel commander Eduard Basurin as saying.

Russian news agency TASS quoted him as saying the pull-back was still being organised and that the actual withdrawal of weapons would take place from Tuesday.

Ukrainian military spokesman Andriy Lysenko said by telephone there was no confirmation yet as to whether the rebels had started pulling back their weapons.

A Reuters witness saw a 20-vehicle convoy of separatist military trucks with anti-aircraft missile systems and howitzers leaving Debaltseve in the direction of Donetsk.

On Saturday, government forces and rebels exchanged nearly 200 prisoners, one of the first moves to implement the peace deal reached on February 12th in the Belarussian capital Minsk after the French, German, Russian and Ukrainian leaders met.

Both sides continue to accuse each other of violating the ceasefire.

Agencies