A bishop who planned a public prayer for the Russian opposition leader Aleksei A. Navalny was detained as he left his house. Two men were arrested for having a photograph of Mr. Navalny in a backpack. Another man who lay flowers at a memorial said he was beaten by police officers for the small act of remembrance.
As thousands of Russians across the country tried to give voice to their grief for Mr. Navalny, who died in a remote Arctic penal colony on Friday, Russian police officers cracked down, temporarily detaining hundreds and placing more than two dozen in jail.
Until Mr. Navalny’s death at the age of 47, many observers had believed that the Kremlin would limit repression until after presidential elections in mid-March, when President Vladimir V. Putin is all but assured a fifth term. But many now fear that the arrests portend a broader crackdown.
“Those who are detaining people are afraid of any opinion that isn’t connected to propaganda, to the pervading ideology,” said Lena, 31, who brought a sticker to the Solovetsky Stone, a monument to victims of political repression in the Soviet Union. “Don’t give up,” read the sticker — part of a message Mr. Navalny once recorded in case of his death.
Someone else placed a copy of Franz Kafka’s “The Trial” at the pediment, while others hung chains of paper cranes, candles, and a photo of Mr. Navalny smiling with fellow opposition leader Boris Nemtsov, who was assassinated in 2015 in the shadow of the Kremlin.
Lena, who gave only her first name for fear of reprisal, started to cry. “They are scared of Navalny in jail,” she said, “they are scared of dead Navalny, they are scared of the people who bring flowers here to the stone.”
She said: “That’s why it is important to continue doing what we are doing, what this man did.”
At least 366 people have been detained in 39 cities across Russia since Mr. Navalny was pronounced dead, with 31 of them ordered to spend up to 15 days in jail, according to OVD-Info, a Russian-based human rights group that tracks arrests. The rest were released after being held for a few hours. About half of those detained were in St. Petersburg, said Dmitri Anisimov, the group’s press secretary.
In Samara, Russia’s ninth-largest city by population, those who came to remember Mr. Navalny were required to have their passports photographed before being allowed to place their flowers in the snow, according to Caution, News, an independent outlet run by a Russian socialite.
Officials have not released Mr. Navalny’s body to his family — the official cause of death remains unclear — and no funeral plans have been announced.
“Grief is a collective action, and any collective action is by definition political,” said Grigory Yudin, a Russian sociologist and research scholar at Princeton University. “In Russia, if a collective activity is not ordered, it is basically prohibited.”
In Surgut, a city in the Khanty-Mansiysk Autonomous Region in Western Siberia, Bakyt Karybaev said he was beaten during a five-hour detention after laying flowers at an impromptu memorial for Mr. Navalny. He told The New York Times in a phone interview that officers hit him on his head with their palms, put a gun to his head and forced him to lie on the floor with his arms outstretched.
“They told me I was a fascist because I support the fascist Navalny,” Mr. Karybaev said. “Then they told me to confess the true reason that I wanted to lay flowers. They asked if I knew to whom the monument was dedicated. I told them it was to those repressed in the Soviet Union.”
Mr. Karybaev was released after signing a warning acknowledging that he would face a criminal inquiry if he did something similar again. He said he was now taking sedatives to try and calm down.
In Moscow, two men were detained on a bridge near the Kremlin where since 2015 activists have maintained a memorial to Mr. Nemtsov, the opposition politician, who was assassinated that year. According to OVD-Info, the two men, Boris Kazadayev and Ilya Povyshev, were questioned by the police, who detained them after finding a photograph of Mr. Navalny in a backpack belonging to one of the men.
And in St. Petersburg, a bishop who was planning to perform a public prayer for the dead in Mr. Navalny’s honor was detained as he left his house on Saturday, then hospitalized after suffering a stroke in police custody. The bishop, Grigory Mikhnov-Vaitenko, planned to conduct the prayer near the city’s Solovetsky Stone, a monument similar to the one in Moscow.
While protests are effectively banned in contemporary Russia, religious leaders are legally allowed to hold services in public without prior consent. Bishop Mikhnov-Vaitenko, a member of the Apostolic Orthodox Church, had published his intention to hold the prayer the day before on his Facebook page and his Telegram channel, which has more than 5,000 followers.
His next post appeared to be a selfie that resembled a mug shot at the police station where he was being held. He was charged with organizing a public gathering that constituted a “violation of public order,” which carries a possible sentence of up to 15 days in jail.
Then late Saturday, an opposition politician, Lev Shlosberg reported that the bishop had been hospitalized following a stroke.
Bishop Mikhnov-Vaitenko, a prominent human rights activist, severed his ties with the Russian Orthodox Church in 2014, after Russia illegally annexed Crimea and fomented a proxy war in Ukraine. The Russian Orthodox Church, the largest religious community in the country, has supported the Kremlin and given its imprimatur to the invasion of Ukraine. On Saturday, its branch in St. Petersburg called on the public to ignore the bishop’s calls for public action in a post on Telegram.
After his detention, the prayer service was conducted by a colleague from the Apostolic Orthodox Church. Video of the event shows several dozen people gathered around the Solovetsky Stone, which was heaped with flowers. Once the service ended, 10 people were detained, according to MR 7. News, a St. Petersburg news outlet.
The severity of the crackdown brought condemnation from Mr. Shlosberg, a veteran Russian opposition politician from the western Pskov region.
“Is the inability to conduct a legal and peaceful religious ceremony a grave or not yet grave enough consequence for society?” he wrote on Telegram, saying that Russians were being denied rights they are entitled to under the Constitution.
“Apparently, the authorities themselves do not understand where the limit of this lawlessness is,” Mr. Shlosberg said. “The intention to suppress any social manifestations, including even natural grief, is leading our country not only into the abyss of lawlessness (there are no longer any rights), but into the abyss of misanthropy.”
As all this was happening, the state media was airing regularly scheduled entertainment shows. News broadcasts showed reports from the Russian front near Avdiivka, the Ukrainian city that fell to occupying Russian forces on Friday, along with figure skaters at the All-Russian Exhibition Center in Moscow. And on Rossiya 1, the country’s flagship show, “News of the Week,” spent much of its time rehashing Tucker Carlson’s interview with Mr. Putin, and the American media personality’s praise for the Moscow public train system.
Alina Lobzina contributed reporting from London.