"We will take back Donbas. We will restore Ukrainian heritage and we will demonstrate that our unity is another crucial factor in our victory," Poroshenko addressed a crowd of people gathered at Kiev's Independence Square (Maidan Nezalezhnosti).
Speaking at the march commemorating victims of Tuesday's bus attack in eastern Ukraine's Volnovakha village, Poroshenko promised not to "yield another inch" of Ukrainian soil.
The Volnovakha bus attack took the lives of 12 people and injured 18 others.
Kiev had previously claimed that the bus had been hit by rockets fired from the east, the direction of a town where local independence forces had been allegedly present at the time. Authorities of the self-proclaimed Donetsk People's Republic (DPR) stressed they had not been involved in the attack.
Despite the ceasefire agreement reached by the warring sides in September 2014, reports of ongoing violations continue to emerge in eastern Ukraine.
