(CNN) -- Hundreds gathered in Kiev's Independence Square on Sunday to honor the victims of violence that has ravaged Ukraine for months, including 49 people killed when a military plane was shot down.
Ukraine President Petro Poroshenko had declared Sunday a day of mourning. The crowd in Kiev observed a moment of silence at noon local time (5 a.m. ET).
"We bow our heads to the heroes who lost their lives for the sake of their country, to prevent the war from coming into the house of each of us," Prime-Minister Arseniy Yatsenyuk said.
"They lost their lives because they defended men and women, children and the elderly who found themselves in a situation facing a threat to be killed by invaders and sponsored by subhumans," he said. "First, we will commemorate the heroes by wiping out those who killed them and then by cleaning our land from the evil."
The Ilyushin-76 military transport plane went down early Saturday while approaching an airport in the eastern Ukrainian city of Luhansk, said military spokesman Vladislav Seleznyov. Ukrinform said the plane was carrying paratroopers, equipment and food.
"All those involved in the cynical act of large-scale terrorism must necessarily be punished," Poroshenko told Ukrinform, the state news agency. "Ukraine needs peace. Terrorists, however, will receive an adequate response."
Ukraine's defense ministry said the plane, which was transporting military personnel, was shot down by insurgents using anti-aircraft machine guns. Ukrinform also said three soldiers were killed Saturday morning near Mariupol.
The attack on the plane, which represents one of the bloodiest single events in that nation's current period of turmoil, spurred a diplomatic flurry to limit further escalation of the crisis.
French President Francois Hollande and German Chancellor Angela Merkel held a telephone discussion Saturday with Russian President Vladimir Putin on the crisis in Ukraine.
Hollande and Merkel expressed their deep concern about the ongoing fighting in eastern Ukraine, particularly Saturday's attack, the Elysee Palace said. The French and German leaders stressed the importance of reaching a cease-fire in Ukraine and avoiding the transfer of fighters and weapons across the Ukrainian-Russian border, according to Hollande's office.
The Kremlin said the three leaders also talked about the drafting of a United Nations Security Council resolution aimed at promoting a settlement. The European leaders stressed the need for an agreement on continuing Russian natural gas deliveries to Ukraine, according to the Elysee.
In a phone conversation with Hollande, Poroshenko urged the European Union to immediately consider sanctions against Russia, including the termination of "military-technical cooperation," Poroshenko's office said.
Ukrainian Prime Minister Arseniy Yatsenyuk spoke by phone with U.S. Secretary of State John Kerry, who expressed condolences for the loss of life and "strong concern" about the flow of heavy weapons and militants across the border from Russia, a senior State Department official said.
Kerry also talked with Russian Foreign Minister Sergey Lavrov, voicing his concern over the downing of the Ukrainian transport plane and pressing Lavrov to "make clear" Russia's commitment to peace, a cease-fire and political dialogue, the state department official said.
Video shows plane shot down
In a statement, Poroshenko said the army and state border service had closed 248 kilometers of Ukraine's border.
"The goal is for us to take back control of Ukraine over the state border and provide adequate response to terrorists," he said.
Video posted to YouTube claimed to show the plane being shot down. Small flashes can be seen climbing skyward, followed by large flashes on the ground on the distant horizon.
The plane was also carrying military machinery and supplies, the ministry said. Officials originally had said the incident occurred late Friday but later updated the time.
In an indication of the continued opposition from some quarters to the Kiev government, the state security office said in a statement Saturday that a homemade bomb was found overnight outside a gate to Poroshenko's office.
The device was made of grenades and a kilogram of other material, such as metal screws, and had a threatening note attached that demanded that the President stop military action against the separatists, the statement said. The person who left it was spotted but fled without detonating it.
The Kiev-based government is carrying out what it calls an anti-terrorist operation, centered in the Donetsk and Luhansk regions, to try to regain control from pro-Russia separatists.
Russia and Ukraine have been engaged in a tense standoff since March, when Russia annexed Crimea and massed troops along its border with Ukraine. Moscow has since begun to withdraw those forces.