Police in Moscow have detained around 20 people protesting Prime Minister Vladimir Putin's government.

The protesters, shouting "down with the police state" and "Russia without Putin," stood arm-in-arm outside the Central Election Commission to demand a free vote.

Putin is running to regain the post of president in a March 4 election. He is expected to win, but is facing the first serious challenge to his 12-year rule.

A broad protest movement has been galvanized by a December parliamentary election that Putin's party won through what appeared to be widespread fraud.

While police have allowed mass demonstrations to take place peacefully in Moscow, they have continued to use force to break up small, unauthorized protests such as the one held Tuesday night.