A spokeswoman for Alexei Navalny confirmed Saturday that the Russian opposition leader had died at a remote Arctic penal colony and said he was “murdered,” but it is unclear where his body is.

An official note handed to Navalny’s mother stated that he died at 2:17 p.m. local time Friday, Kira Yarmysh said. She added that an employee of the prison colony said that Navalny’s body was taken to the nearby city of Salekhard as part of a probe into his death.

When a lawyer and Navalny’s mother visited the morgue at Salekhard, it was closed, Navalny’s team said, writing on their Telegram channel. The lawyer called the morgue and was told that Navalny’s body is not there, his team said. 

Another of Navalny’s lawyers went to Salekhard’s Investigative Committee and was told that the cause of Navalny’s death has not yet been established and that new investigations are being done with the results to be released next week, Ms. Yarmysh said.

“It’s obvious that they are lying and doing everything they can to avoid handing over the body,” she wrote on X, formerly Twitter, adding that his team “demand that Alexei Navalny’s body be handed over to his family immediately.”

Russia’s Federal Penitentiary Service reported that Navalny felt sick after a walk and became unconscious at the penal colony at the town of Kharp, in the Yamalo-Nenets region about 1,200 miles northeast of Moscow. An ambulance arrived, but he couldn’t be revived. The cause of death is still “being established,” it said.

The head of the board of Navalny’s Anti-Corruption Foundation, Maria Pevchikh, said that the opposition leader would “live on forever in millions of hearts.”

“Navalny was murdered. We still don’t know how we’ll keep on living, but together, we’ll think of something,” she wrote on X.

Arrests continued Saturday after more than 100 people were detained in various Russian cities Friday when they came to lay flowers in memory of Navalny at memorials to the victims of Soviet-era purges, according to OVD-Info, a group that monitors political repression in Russia.

The tributes were removed overnight, but people continued trickling in with flowers on Saturday. In Moscow, a large group of people chanted “shame” as police dragged a screaming woman from the crowd, video shared on social media showed.

More than 10 people were detained at a memorial at St. Petersburg, including a priest who came to conduct a service for Navalny there.

In other cities across the country, police cordoned off some of the memorials and officers were taking pictures of those who came and writing down their personal data in a clear intimidation attempt.

“After the murder of Alexei Navalny, it’s absurd to perceive Putin as the supposedly legitimate head of the Russian state. He is a thug who maintains power through corruption and violence,” President Zelensky said at the Munich Security Conference in Germany Saturday.

“Just yesterday he tried to send us all a clear message — as the Munich Security Conference opened, Putin murdered another opposition leader,” Mr. Zelensky said.

Britain’s foreign secretary, David Cameron, said Saturday that Britain “will be taking action” against the Russians responsible for the death of Alexei Navalny.

Speaking to broadcasters in Munich, Mr. Cameron said “there should be consequences” for “appalling human rights outrages like this.” He said Britain would “look at whether there are individual people that are responsible and whether there are individual measures and actions we can take.” Mr. Cameron did not say whether the response would consist of financial sanctions or other measures.

President Biden Friday 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.

The Kremlin bristled Friday at the outpouring of anger from world leaders, with Putin’s spokesman, Dmitry Peskov, saying the statements are “unacceptable” and “outrageous,” noting that medics haven’t issued their verdict on the cause of Navalny’s death.

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, and received a sentence of 19 years for extremism.

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.”

The news of Navalny’s death comes less than a month before an election that is expected to give Mr. Putin another six years in power.

It shows “that the sentence in Russia now for opposition is not merely imprisonment, but death,” a former British ambassador to Belarus and senior fellow for Russia & Eurasia at the International Institute for Strategic Studies at London, Nigel Gould-Davies, said.

Hours after Navalny’s death was reported, his wife, Yulia Navalnaya, made a dramatic appearance at the Munich conference.

She said 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.”