Alexei Navalny, who crusaded against official corruption and staged massive anti-Kremlin protests as President Vladimir Putin's fiercest foe, died Friday in the Arctic penal colony where he was serving a 19-year sentence, Russia's prison agency said. He was 47.

The stunning news — less than a month before an election that will give Putin another six years in power — brought renewed criticism and outrage from world leaders toward the Russian president who has suppressed opposition at home.

After initially allowing people to lay flowers at monuments to victims of Soviet-era repressions in several Russian cities, police sealed off some of the areas and started making arrests.

More than 100 people were detained in eight cities, including Moscow, St. Petersburg, Murmansk in the Arctic Circle, Krasnodar and Rostov-on-Don in the south of Russia, according to the OVD-Info monitoring group. Shouts of "Shame!" were heard as Moscow police rounded up more than a dozen people — including one with a sign reading "Killer" — near a memorial to political prisoners, the group said.

But there was no indication Navalny's death would spark large protests, with the opposition fractured and now without its "guiding star," as an associate put it.

Russia's Federal Penitentiary Service reported Navalny felt sick after a walk Friday and lost consciousness at the penal colony in the town of Kharp, in the Yamalo-Nenets region about 1,900 kilometers (1,200 miles) northeast of Moscow. An ambulance arrived, but he couldn't be revived; the cause of death is "being established," it said.

Navalny had been jailed since January 2021, when he returned to Moscow to face certain arrest after recuperating in Germany from nerve agent poisoning he blamed on the Kremlin. He was later convicted three times, saying each case was politically motivated.

After the last verdict, Navalny said he understood he was "serving a life sentence, which is measured by the length of my life or the length of life of this regime."

Hours after his death was reported, Navalny's wife, Yulia Navalnaya, made a dramatic appearance at a security conference in Germany where many leaders had gathered.

She said she had considered canceling, "but then I thought what Alexei would do in my place. And I'm sure he would be here," adding that she was unsure if she could believe the news from official Russian sources.

"But if this is true, I want Putin and everyone around Putin, Putin's friends, his government to know that they will bear responsibility for what they did to our country, to my family and to my husband. And this day will come very soon," Navalnaya said.

Praise for Navalny's bravery poured in from Western leaders and others opposing Putin. Navalny's health has deteriorated recently and the cause of death may never be known, but many of them said they held Russian authorities ultimately responsible — particularly after the deaths of many Kremlin foes.

U.S. President Joe Biden said Washington doesn't know exactly what happened, "but there is no doubt that the death of Navalny was a consequence of something Putin and his thugs did."