A blast ripped through a crowd of thousands marching for peace in eastern Ukraine, killing at least two and injuring as many as 11 people, the country's Interior Ministry said in a statement.

The explosion, which is being investigated as an "assassination," occurred near a metro station in Kharkiv, Ukraine's second largest city with some 1.4 million people, at 1:20 p.m. local time, as a massive crowd of participants marched by, police said.

The video below appears shows the moment of the explosion. (Warning: Graphic content.)

One police officer was among those killed; four police officers were among the wounded, authorities reported.

Forensic experts work at a site of an explosion, as a body of a victim is covered with a Ukrainian flag in Kharkiv, Ukraine Sunday, Feb. 22, 2015.

While details of the attack remain murky, Security Service of Ukraine (SBU) spokesperson Markian Lubkivsky reported that four suspects in what he called the "terrorist act" had been apprehended.

He told Ukraine's 112 Television that the suspects had received help from Russia in carrying out the attack.

"They are Ukrainian citizens who underwent instruction and received weapons in the Russian Federation, in Belgorod," Lubkivsky said, referring to a nearby Russian city across the border.

Russia did not immediately respond to the allegations.

Lubkivsky said the group had planned further attacks in Kharkiv. With that in mind, National Security and Defense Council secretary Oleksandr Turchynov put the city on heightened alert, launching an anti-terror operation.

The rally, dubbed the March of Dignity by President Petro Poroshenko, who took part in a similar event in the Ukrainian capital Kiev at the same time, began at noon local time, with thousands of pro-Ukrainian participants waving the country's blue-and-yellow flag and holding placards of protesters cut down by police bullets one year ago during the Euromaidan revolution in Kiev.

#Харків зараз: на одній сцені представники різних конфесій: православні, католики, греко-католики, мусульмани. pic.twitter.com/A05aptuHNy — IT Sector Харьков (@itsector) February 22, 2015

Sunday's attack follows a series of mysterious blasts in cities around Ukraine, some of which have been deadly. Authorities believe the attacks are meant to sow panic and chaos across a country already gripped by war.

"This is the kind of reality that [Ukrainians] will live with during the next few years," Olexiy Melnyk, co-director for Foreign Relations and International Security Programs at the Kiev-based Razumkov Center, told Mashable about the spate of attacks in December.

Ukrainian and American officials in Kiev were quick to post messages of support after the attack, with the country's foreign minister, Pavlo Klimkin, writing on Twitter, "The terrorist attack in Kharkiv says one thing: nothing is sacred. Murder on Forgiveness Sunday. And we didn't break down. Everyone together experiencing deaths and suffering."

Про теракт у Харкові скажу одне: нічого святого. Вбивство у Прощену неділю. Та нас не зламати. Всі разом переживаємо ці смерті і страждання. — Pavlo Klimkin (@PavloKlimkin) February 22, 2015

U.S. Ambassador to Ukraine Geoffrey Pyatt, who joined thousands at the peace rally in Kiev, said the best response to the attack was to continue the march.