The Russian delegation was not present in the General Assembly Hall for Poroshenko's speech, an apparently deliberate boycott. On Monday, the Ukrainian delegation pointedly left the hall when Russian President Vladimir Putin started speaking.
Though he did not mention Putin by name, Poroshenko openly mocked the Russian president's call for an anti-terrorism coalition to fight radicals in Syria, characterizing it as "double-tongued."
"Cool story," he said, his voice dripping in sarcasm. "But really hard to believe.
"How can you urge an anti-terrorism coalition if you inspire terrorism right in front of your own door? How can you talk peace and legitimacy if your policy is war via puppet government? How can you speak for freedom for nations if you punish your neighbor for this choice? How can you demand respect for all if you don't have respect for anyone?"
Many Ukrainians fear their confrontation with Russia, which began in 2014 when Russia annexed Crimea, has been sidelined under a blizzard of international crises, particularly the war raging in Syria.
Ukraine and the United States both say they have proof of regular Russian army troops operating within eastern Ukraine, supporting Russian separatists who have declared independence in the Luhansk and Donetsk regions. Russia denies having troops there, though many reporters have spotted them and talked to them personally.
Poroshenko said Russia had ordered its soldiers operating inside Ukrainian territory to remove the military insignia from their uniforms and identifying marks from their military equipment. He said Moscow had abandoned its soldiers captured on the battlefield, "and cynically used mobile crematorium to eliminate traces of its crime on Ukrainian soil."
He estimated Russian troops and the separatists they back now occupy about 17,000 square miles of Ukrainian territory, causing 1.5 million people to flee to safer regions as the warring parties do battle.
"We are dealing here with a desire to return to the imperial times with a sphere of influence, in a desperate attempt to obtain self-affirmation at others expense," he said, adding that Russia was financing "terrorists and mercenaries."
Poroshenko noted a Russian military build-up in Syria, and posed the rhetorical question, "What and who is next?"
In his remarks to the U.N. on Monday, Putin briefly mentioned the conflict in Ukraine, which he described as a civil war caused by a "military coup…orchestrated from outside."
Poroshenko accused Russia of wielding its Security Council veto against resolutions on Crimea and the Malaysian Airlines flight shot down over Ukraine as a "license to kill," and urged the U.N. to prohibit vetoes on votes over mass atrocities.
Carol Morello is the diplomatic correspondent for The Washington Post, covering the State Department.
Ukrainian President Petro Poroshenko sought to return his country’s long-running conflict with Russia to center stage Tuesday, telling the United Nations General Assembly that Russia has been waging an aggressive war of occupation against Ukraine.