World leaders and Russian opposition activists have spoken with one voice in blaming President Vladimir Putin for the death of imprisoned opposition leader Alexei Navalny.

Australian political leaders have labelled Alexei Navalny a “courageous force for democracy” and made clear who they hold responsible for the Russian opposition leader’s reported death.

Prime Minister Anthony Albanese on Saturday shared his sadness after Russian authorities claimed Mr Navalny died at the “Polar Wolf” Arctic penal colony where he was serving a three-decade sentence.

“Australian mourns the tragic death of Alexei Navalny, a courageous force for democracy in Russia,” Mr Albanese wrote on the social media platform X.

Ukrainian President Volodymyr Zelenskiy said “it is obvious that he was killed by Putin,” said , who was visiting Germany for the Munich Security Conference as he sought aid for his country’s efforts to fight off an invasion by Russia.

“Putin doesn’t care who dies – only for him to hold his position. This is why he must hold on to nothing. Putin must lose everything and be held responsible for his deeds,” Zelenskiy added.

US President Joe Biden said Washington does not know exactly what happened, “but there is no doubt that the death of Navalny was a consequence of something Putin and his thugs did.”

Outspoken Kremlin critic Navalny has reportedly collapsed and died at the age of 47 while serving a lengthy jail term in a harsh penal colony in the Arctic Circle.

The Russian prison service said Navalny “felt unwell” after a walk and collapsed and lost consciousness.

“All necessary resuscitation measures were carried out, which did not yield positive results,” said the prison service.

“Doctors of the ambulance stated the death of the convict.”

There has been no independent verification of Navalny’s death, which comes the month before Russian President Vladimir Putin faces re-election in March.

But the news quickly sparked grief, disbelief, anger and defiance as supporters gathered to lay flowers and blame Putin.

Sentenced to 30 years

Putin’s nemesis and Russia’s most prominent opposition leader was being held at the ‘Polar Wolf’ penal colony about 1900km east of Moscow where he was serving a three-decade sentence.

His mother Lyudmila Navalnaya said her son had been “alive, healthy and happy” when she saw him in prison on February 12, reported Russian newspaper Novaya Gazeta.

“I don’t want to hear any condolences,” she reportedly said.

Navalny’s German lawyer said the prisoner had seemed fit and well as recently as this week when he participated in yet another court hearing.

US President Joe Biden said he was “both not surprised, and outraged” as he paid tribute to Navalny’s bravery and principles.

Biden quickly blamed Russia’s president “and his thugs” for Navalny’s death, saying: “Make no mistake, Putin is responsible.”

The US president said Navalny “bravely stood up to the corruption, the violence, and all the bad things that the Putin government was doing.

‘Putin had him poisoned’

“In response, Putin had him poisoned, he had him arrested, he had him prosecuted for fabricated crimes.

“Even in prison, he was a powerful voice for the truth,” said Biden.

“He could have lived safely in exile after the assassination attempt on him in 2020, which nearly killed him,” he said.

“Instead, he returned to Russia — knowing he’d likely be imprisoned, maybe killed if he continued his work. But he did it anyway. Because he believed so deeply in his country, in Russia.”

There is grief, disbelief and anger after news of Alexei Navalny’s death. Photo: Getty

Navalny survived an attempt to poison him with a nerve agent in Siberia in August 2020. After receiving treatment in Germany he then bravely chose to return to Russia in 2021 to face arrest and imprisonment.

In an interview in Moscow in 2011, Navalny was asked by Reuters if he was afraid of challenging Putin’s system.

“That’s the difference between me and you: you are afraid and I am not afraid,” he said. “I realise there is danger, but why should I be afraid?”

Kremlin spokesman Dmitry Peskov said Putin, who is running for re-election in a month, had been told about Navalny’s death.

Supporters of Navalny said they could not confirm he was dead, but that if he was then they believed he had been killed.

“Russian authorities publish a confession that they killed Alexei Navalny in prison,” Navalny aide Leonid Volkov wrote on social media.

“We do not have any way to confirm it or to prove this isn’t true.”

Navalny’s lawyer was on his way to the Polar Wolf prison in Kharp where Navalny died.

Russian state television barely mentioned the reformer’s death, broadcasting instead a press conference by the central bank.

Alexei Navalny was the most outspoken critic of Russian President Vladimir Putin. Photo: Getty

Russian newspaper editor and Nobel Peace Prise laureate Dmitry Muratov said Navalny’s death was “murder”, and said that he believed harsh treatment had led to his demise.

“My sincere belief is that it was the conditions of detention that led to Navalny’s death … His sentence was supplemented by murder,” Muratov said, offering condolences to his family.

Condemnation quickly poured in from Western governments.

“Dreadful news about Alexei Navalny … If the report about his death in Russian prison is true it represents another terrible crime by Putin’s regime,” Swedish Foreign Minister Tobias Billstrom said on X.

“The ruthlessness against Navalny shows again why it is necessary to continue to fight against authoritarianism.”

British Prime Minister Rishi Sunak said: “This is terrible news. As the fiercest advocate for Russian democracy, Alexei Navalny demonstrated incredible courage throughout his life.”

Nato secretary general Jens Stoltenberg said he was “deeply saddened and disturbed” by the reports of Navalny’s death.

“We need to establish all the facts, and Russia needs to answer all the serious questions about the circumstances of his death.”

-with AAP