President Vladimir Putin has signed a law letting him pick candidates to lead Russia's regions if local lawmakers decide to scrap popular polls.

The law allows each of the country's 83 regions to repeal direct elections of governors, introduced just last year in a concession during a wave of protests by Russians fed up with Putin's dominance and demanding a stronger political voice.

Mr Putin has said the law is needed to protect the rights of minorities in ethnically-mixed regions such as the mostly Muslim provinces of the insurgency-plagued North Caucasus.

The Kremlin is concerned that direct elections in the volatile regions could spark unrest or involve candidates whose loyalty is in question. Russia is holding the Winter Olympics next February in Sochi, close to the North Caucasus provinces.

But critics of the president say the law is a setback to democracy and favours the ruling United Russia party, which is less popular than Mr Putin himself and had its parliamentary majority reduced in a December 2011 election.

They fear the Kremlin and United Russia will use the law to sideline opposition candidates.

"It's another lever to manage everything from Moscow," said Boris Nemtsov, a prominent opposition leader and a former cabinet minister in the 1990s under Putin's predecessor, Boris Yeltsin.