Alexey Navalny, imprisoned Russian opposition leader and outspoken critic of  President Vladimir Putin, died in prison Friday, according to Russian officials.

Officials said Navalny, 47, felt unwell and lost consciousness after taking a walk Friday. An ambulance arrived, but resuscitation attempts were unsuccessful, they said. The cause of death was "being established." Navalny was serving a 19-year sentence.

World leaders, including Ukrainian President Volodymyr Zelenskyy, quickly blamed the reported death on Putin. U.S. Vice President Kamala Harris said Navalny's death, if confirmed, is further sign of Russia's brutality and that Russia is responsible no matter what story it tells.

Navalny founded the Anti-Corruption Foundation and was a 2010 Yale World Fellow. The fellowship convenes disruptive, innovative leaders for four months at the university "to grow intellectually, share knowledge, strengthen skills and expand networks," according to its website.

Navalny dedicated his life to open society and a better Russia, according to a statement released Friday by Emma Sky, director of Yale's International Leadership Center.

"After being poisoned by the state, he bravely returned to Russia, only to face immediate arrest. He defied a dictatorship and paid for it with his life," the statement said.

Sky said Friday, "Alexey was imprisoned in inhumane conditions on trumped-up charges but never lost the ability to laugh — and make others laugh. He held onto his human dignity and showed that the regime could not break him.

"Only a few weeks ago, after he was taken on a grueling three-week trip to an Arctic penal colony, he resurfaced with humorous tweets — and reportedly made a judge laugh by asking if his previous jail held a naked karaoke party to celebrate his departure."

Navalny was poisoned with a nerve agent in 2020 and blamed the Kremlin, which denied the accusation. Several Yale World Fellows signed a statement at the time saying they admired Navalny's courage to speak out against corruption.

"We call on the Russian Government to allow and support an independent, transparent and credible investigation to bring the perpetrators of this horrific crime to justice," the statement said. "In every society, all citizens — including opposition leaders such as Alexey Navalny — should be protected by the law."

Navalny was a "larger-than-life figure," with a mischievous grin, sense of humor and devotion to family, the statement Friday said. His unflagging courage and selfless sacrifice will be remembered, the statement said.

An empty seat was saved for Navalny in the front row at the World Fellows' 20th reunion last year, the statement said.

"He will no longer be able to occupy it, but a place remains for all those around the world who continue in his path," it said. "His spirit is uncrushable. He lives on in all of us."

Associated Press reports are included in this story.