Live updates: follow the latest news on Russia-Ukraine
He said the Donbas has been a focus for Russia and now “troops are destroying it as if they wanted only stones left, without any people at all".
“These 50 days showed me world leaders in a different way," Mr Zelenskyy said in his nightly video address on Instagram.
"The countries are different. I have seen great generosity from those who are not rich themselves. I have seen extraordinary determination by those who world leaders usually didn’t take seriously.
"I have seen that the democratic world has indeed those whose will is enough to defend freedom from attacks of tyranny."
He also had words of praise for his people, "those who showed that Russian ships can only sink to the bottom".
He said that during the 50 days of war, "Ukraine became a hero for the whole free world and for those who have the courage to call a spade a spade. For those who are not poisoned by propaganda.
"You have all become heroes. All Ukrainian men and women who withstood and do not give up. And who will win. Who will return peace to Ukraine. I'm sure of it," Mr Zelenskyy said.
Ukraine’s leader also called for tougher sanctions against Moscow after the consequences of its invasion.
“Russian troops repeat on our land what Europe probably only saw during World War II,” Mr Zelenskyy said.