President Vladimir Putin condemned the terrorist attack on the Crocus City Hall in Russia's capital Moscow as a "purposeful massacre," as he declared a day of mourning.

"The criminals, purposefully and in cold blood killed, shot our citizens and our children at point-blank range," Putin said in a televised address to the nation on Saturday. "Like the Nazis, who once carried out massacres in the occupied territories, they planned to arrange a demonstrative execution, a bloody act of intimidation."

In the wake of the attack that killed at least 115 people, the president declared Sunday, March 24, as the day of national mourning. "I express my deep, sincere condolences to all those who lost their loved ones. The whole country and our entire people are grieving with you," he said.

Putin said all four people directly involved in committing the attack were arrested on their way to Ukraine, where "a window" was prepared for their escape.

"All four direct perpetrators of the terrorist attack, all those who were shooting and killing people, were found and detained. They tried to hide and moved towards Ukraine, where, according to preliminary data, a window was prepared for them from the Ukrainian side to cross the state border. A total of 11 people were detained," he said.

Russian special services are working to identify accomplices of assailants at Crocus City concert hall in Moscow region, he added.

Gunmen opened fire as rock band Picnic was about to perform at the concert venue just west of Moscow. The terrorists also used flammable liquid to set the hall on fire, which quickly spread to a huge area of almost 13,000 square meters.