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Those involved in such "cynical acts of terrorism" must be punished, said Mr Poroshenko, who summoned security officials for an emergency meeting.
The defence ministry said the Ilyushin-76 transport plane came under anti-aircraft fire over the city of Luhansk.
It was about to land there, carrying troops and military equipment.
It is thought to be the biggest loss of life suffered by government forces in a single incident since the Ukrainian government in Kiev began an operation to try to defeat the insurgency in the east.
Separately, Moscow has reacted angrily to a violent protest outside its embassy in Kiev, which saw windows
Russia's foreign ministry accused Ukrainian police of doing nothing to halt the attack, saying it was a "grave violation of Ukraine's international obligations", and demanded Kiev take steps to ensure the safety of its diplomats.'Adequate response'
Ukrainian defence ministry officials are investigating how the separatists brought down the plane in the east, with reports suggesting the use of three "Stinger-like" missiles or a large-calibre machine-gun.
Everyone on board - nine flight crew and 40 troops - was killed.
In a statement released later on Saturday, Mr Poroshenko said: "Ukraine needs peace. However, the terrorists will receive an adequate response."
Rebel fighters were seen combing through the charred wreckage of the plane, south-east of Luhansk, on Saturday.
"This is how we work," one of them, who identified himself as Pyotr, told Reuters news agency. "The fascists can bring as many reinforcements as they want but we will do this every time."
The eastern regions of Donetsk and Luhansk declared independence last month. In March Russia annexed the Crimean peninsula in the wake of the removal of Ukraine's pro-Moscow President Viktor Yanukovych.
