More than a thousand Russians have rallied in Saint Petersburg after a century-old bas relief of a mythical demon was destroyed amid fears of increasing religious intolerance under President Vladimir Putin.

The protest came on Sunday, a week after the figure of Mephistopheles was ripped down from the facade of a century-old building in Saint Petersburg.

An obscure group calling itself the Cossacks of Saint Petersburg claimed responsibility in an open letter, saying the figure encouraged "open worship of Satan" and was unacceptable because it was opposite a church.

Police said they had found smashed fragments of the figure in rubbish sacks.

Prosecutors have opened an investigation into the destruction of cultural heritage, which carries a jail term of up to two years.

The demonstrators, including architecture preservationists, gathered in front of the building on Sunday, calling the destruction a "brazen act of vandalism".

"Hands off art," read one placard, while another one said in English: "Save our Saint Petersburg."

"What happened is awful," said Anna Astakhova, 35. "If it's true that the bas relief was destroyed for religious reasons, then we are descending into the Middle Ages. This is inadmissible."

Another protester, Galina Vanina, added: "I am an Orthodox Christian myself but I do not support this absurdity."

Meanwhile, a spokesman for the powerful Russian Orthodox Church said the attack was an understandable reaction.

"Mephistopheles embodies evil in this world and this person decided to act, most likely, to kill Evil," spokesman Roman Bagdasarov told pro-Kremlin daily Izvestia.