Moscow: Alexei Navalny's supporters on Saturday accused Russian authorities of being "killers" who were "covering their tracks" by refusing to hand over his body, as the Kremlin stayed silent despite Western accusations and a flood of tributes to the late opposition leader.

The 47-year-old Kremlin critic died in an Arctic prison on Friday after spending more than three years behind bars, prompting outrage and condemnation from Western leaders and his supporters.

His death, which the West has blamed on the Kremlin, deprives Russia's opposition of its figurehead just a month before elections poised to extend President Vladimir Putin's grip on power.

On Saturday, Navalny's mother, Lyudmila, and his lawyer were refused access to his body after arriving at the remote Siberian prison colony where he had been held, his spokeswoman Kira Yarmysh said.

"It's obvious that the killers want to cover their tracks and are therefore not handing over Alexei's body, hiding it even from his mother," Navalny's team said in a post on Telegram.

"They don't want whatever method they used to kill Alexei to come out," Yarmysh said in an online broadcast, in his backers' strongest accusation yet of foul play.

Across the country, Russian police on Saturday moved swiftly to break up small protests in honour of the Kremlin critic, arresting more than 400 people in 36 cities, the OVD-Info rights group said.

"Alexei Navalny's death is the worst thing that could happen to Russia," said one note left among the flowers at a makeshift memorial in Moscow.

Silence from Kremlin
After initially pushing back at accusations they were to blame, there was no comment from the Kremlin on his death on Saturday, despite an angry chorus of condemnation from Western leaders.

G7 foreign ministers meeting in Munich held a minute's silence for the leader on Saturday, while US President Joe Biden explicitly blamed Putin. Putin, 71, has not commented.

In the past, on the rare occasions when he has been asked about his most vocal critic, the Russian leader famously avoided saying Navalny's name.

Speaking at the Munich Security Conference hours after news of her husband's death, Yulia Navalnaya said Putin and his entourage would be "punished for everything they have done to our country, to my family and to my husband".

She called on the international community to "unite and defeat this evil, terrifying regime".

Russian Nobel Peace Prize winner Dmitry Muratov said Navalny's death was "murder" and that he was "tortured and tormented" for three years in prison.

Tributes continued to pour in on Saturday, as supporters staged anti-Putin protests and pop-up tributes to Navalny around the world.

In a video posted by the independent Sota outlet from Moscow, a woman could be heard screaming as a crowd of police officers detained her, to chants of "shame" from onlookers.

On a bridge next to the Kremlin, hooded men were seen scooping up flowers into bin bags that had been laid at an unofficial memorial to Navalny ally and slain Kremlin critic Boris Nemtsov.

Russian courts on Saturday started issuing short-term jail sentences of up to 15 days for those detained at the commemorations, rights groups reported.

Navalny died on Friday when he lost consciousness after having "felt bad after a walk", Russia's federal penitentiary service said.

One of his lawyers, Leonid Solovyov, told the Novaya Gazeta newspaper that Navalny was "normal" when another lawyer saw him on Wednesday.

In footage of a court hearing from his prison colony on Thursday, the day before his death, Navalny was seen smiling and joking as he addressed the judge by video link.

Navalny's mother and lawyer were told on Saturday he died of "sudden death syndrome" -- a vague term with no specific medical meaning.

"There's no such thing ... that can't be the cause of death," his spokeswoman Yarmysh said.

Anti-corruption campaigns
Navalny, who led street protests for more than a decade, became a household name through his anti-corruption campaigning and electric charisma.

His exposes of official corruption, posted on his YouTube channel, racked up millions of views and brought tens of thousands of Russians onto the streets, despite harsh anti-protest laws.

He was jailed in early 2021 after returning to Russia from Germany, where he was recovering from a near-fatal poisoning attack with Novichok, a Soviet-era nerve agent.

A subsequent investigation by his team and several media outlets said a Russian FSB hit squad was behind the attack.

Upon his return he was hit with a barrage of charges, including a 19-year prison sentence for "extremism", widely condemned by rights groups and seen in the West as retribution for his opposition to the Kremlin.

His decision to go back despite knowing he would face jail brought him global admiration.

"I'm not afraid and I call on you not to be afraid," he said in an appeal to supporters as he landed in Moscow, moments before being detained.

His arrest spurred some of the largest demonstrations Russia had seen in decades, and thousands were detained at rallies nationwide calling for his release.

300 days in solitary confinement
From behind bars, Navalny became a staunch opponent of Moscow's full-scale military offensive against Ukraine. He was forced to watch on, helplessly, as the Kremlin dismantled his organisation and locked up his allies.

Dozens of his top supporters fled into exile and continued to campaign against the offensive on Ukraine and repression inside Russia.

Late last year, Navalny was moved to a remote Arctic prison colony nicknamed "Polar Wolf" in Russia's Yamalo-Nenets region in northern Siberia.

He said in January that his daily routine included prison walks in freezing temperatures.

Since being jailed, he spent more than 300 days in solitary confinement, where prison authorities kept him over alleged minor protocol infringements.

In a documentary filmed before he returned to Russia, Navalny was asked what message he wanted to leave to the Russian people should he die or be killed.

"Don't give up. You mustn't, you can't give up," he said.

"All it takes for evil to triumph is for good people to do nothing. Therefore, don't do nothing."