Pro-Kremlin activists have trashed a memorial to slain opposition politician Boris Nemtsov, which was constructed on the Moscow bridge where he was shot dead in February.

Vandals smashed a framed photograph of Nemstov that mourners had left at the scene, wrote messages denouncing him as a "traitor", and defaced a sign left by supporters renaming the site Nemtsov Bridge in honour of the Putin critic, colleague Ilya Yashin wrote on Facebook, reports Radio Free Europe.

On Russian social networking site VKontakte, activists from the fringe pro-Kremlin group SERB declared the group's determination to "always fight traitors to Russia in all its manifestations", and claimed responsibility for the incident.

Pictures posted online on 24 March show members of the group standing by the memorial, holding broken parts of the Nemtsov Bridge tribute.

Supporters repaired the memorial, delivering fresh flowers, according to social media posts.

"Borya [Nemtsov], you have not died, you continue even from the grave to fight along with us against the [rot] that has seized the country," wrote supporter Mark Galperin on Facebook, reports the Moscow Times.

The mayor of Moscow said that the site does not have official memorial status so cannot receive official protection, reports

Nemtsov was shot dead on the Bolshoy Moskvoretsky Bridge, within sight of the Kremlin, on 27 February.

Two Chechens have been charged with the killing, within one alleging that he was tortured into confessing.

In public speeches Russian President Vladimir Putin has denounced a "fifth column" of "national traitors" who have betrayed Russia, with Russian state media regularly alleging plots by shadowy agents in conjunction with Russian supporters to undermine the country.