A volunteer guard at the makeshift memorial to opposition leader Boris Nemtsov, who was gunned down near the Kremlin in 2015, has died in the hospital after being punched by an unknown assailant.

Police have opened an investigation into the death of Ivan Skripnichenko, 35, who passed away in a local hospital on 23 August.

State news agency RIA Novosti reported the tentative cause of death was cardiomyopathy, but acquaintances of Mr Skripnichenko have linked it to the incident at the memorial.

The impromptu grouping of flowers, candles and portraits on the bridge where Mr Nemtsov was killed has been attacked by pro-Kremlin activists and cleared by city authorities numerous times.