"Alexey, we will never forget you," Polish Prime Minister Donald Tusk said in a post on the X social media platform.
"And we will never forgive them," Tusk added.
Polish Foreign Minister Radosław Sikorski took to X to offer his condolences to Navalny's family.
He said he was grieving with "democrats in Russia and beyond" over Navalny's death. He called Navalny a "hero."
"My heart is now with the family of Alexei Navalny and all the democrats for whom he is a hero," Sikorski said in a post.
He added that Navalny had "challenged" Russian President Vladimir Putin and "did nothing wrong."
The Russian opposition leader "was convicted on false charges and imprisoned in terrible conditions, for which Putin is responsible," Sikorski also said.
US Vice President Kamala Harris said on Friday that Navalny's death, if confirmed, would be further proof of the brutality of Putin, the Reuters news agency reported.
Reports of Navalny's death shook the annual Munich Security Conference where leaders gathered to bolster unity against Russia's nearly two-year-old invasion of Ukraine, according to Reuters.
"Let us be clear, Russia is responsible," Harris said.
US Secretary of State Antony told the conference in Germany that if reports of Navalny's death are true, they testify to Russia's "weakness and rot," Reuters reported.
Russian opposition leader Alexei Navalny inside a glass cage prior to a hearing in a Moscow court on Feb. 16, 2021. Photo: EPA/BABUSHKINSKY DISTRICT COURT PRESS
Russian authorities said Navalny died after a fall at an Arctic penal colony on Friday.
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Source: PAP, IAR, Reuters