- Police make no attempt to break up the unauthorized rally
- Several hundred people gather for the protest in central Moscow
- Sergey Udaltsov is an opposition leader who is staging a hunger strike
- He has taken part in protests against election results
Hundreds of demonstrators gathered in central Moscow on Thursday to protest the detention of Russian opposition activist Sergey Udaltsov.
About 3,000 people said they might attend ahead of the event, according to its Facebook page, although Udaltsov's wife Anastasia said city hall had refused to give permission for the rally.
Reporters at the scene estimated that several hundred protesters had turned out. The mood was calm, with no attempt by police to break up the rally despite its unauthorized status.
That may reflect an apparent change in approach by Prime Minister Vladimir Putin, who was initially dismissive of protesters but has in recent days struck a more conciliatory tone.
Udaltsov, the leader of the opposition Leftist Front movement, had his 15-day detention extended by another 10 days on Sunday, Russia's RIA Novosti news agency said.
He was arrested on December 4 for not serving out the full term of his previous administrative detention and escaping from a hospital, according to the news agency.
Udaltsov is staging a hunger strike in protest of his detention and has been hospitalized twice during it, RIA Novosti reported. He was taken to the hospital again after his term was extended.
Tens of thousands of Russians have taken to the streets to protest the December 4 parliamentary election results amid claims of fraud. Rights groups say hundreds of people have been detained over the protests and have called for claims of police mistreatment to be investigated.
Putin's ruling United Russia Party received 49.5% of the vote, down from 64% four years ago, in an election widely viewed as marred by vote rigging and ballot-box stuffing. It kept Putin's party in power, although in diminished form.
Putin intends to run in the country's presidential elections in March.