Mourners mark Alexei Navalny's birthday at graveside memorial

Dozens of Alexei Navalny supporters have visited his grave to lay flowers and pay tribute to the late Kremlin critic on what would have been his 48th birthday. 

The opposition leader died at an Arctic prison colony in February, prompting outrage from Western governments.

Nearly four months on, his family say the cause of his death remains unexplained.

His widow Yulia Navalnaya has accused Vladimir Putin of ordering his murder, and last week his allies called for additional sanctions to punish the Russian president's inner circle.

The Kremlin has repeatedly denied any involvement in his death. According to his death certificate, he died of natural causes.

Navalny's mother, Lyudmila, as well as his mother-in-law Alla Abrosimova, were among those who gathered at his grave in the Borisovskoye cemetery in southeast Moscow on Tuesday.

Video posted by SOTAvision on the social media platform Telegram shows a memorial service led by priest Dmitry Safronov marking 40 days since the activist's death.

Mr Safronov was previously banned from clerical duties by the Russian Orthodox Church for presiding over a similar service in March.

According to the independent Novaya Gazeta newspaper, three police officers were on duty near the cemetery but they did not make any arrests.

Having been convicted of multiple charges, ranging from fraud to extremism, Navalny was serving sentences totalling more than 30 years when he died.

His Anti-Corruption Foundation (FBK) is outlawed in Russia, and has been accused by the authorities of having links to the CIA. What remains of his team now operates in exile.