Hundreds of people on Thursday demonstrated in a square in central Moscow against the continued detention of left-wing activist Sergei Udaltsov, an AFP correspondent reported. Demonstrators chanted \"freedom for political prisoners!\", but organisers warned the demonstrators through loudspeakers to avoid shouting political slogans and showing banners. They said the action had not been agreed with the police and they wanted to avoid arrests ahead of the New Year. Police also warned the demonstrators through their own megaphones that \"this demonstration is not sanctioned\". The rally was taking place amid a tense atmosphere on Pushkin Square with dozens of police buses parked around the square to form a security barrier and security forces keeping a careful eye on proceedings. But so far there was no unrest or arrests and the protesters were politely applauding the speakers and obeying their pleas to avoid provoking the situation. The plight of Udaltsov -- sentenced to three short successive prison terms in recent weeks despite poor health due to a hunger strike -- became a rallying cause for the opposition after their mass protests against Vladimir Putin. \"I am here to support Sergei Udaltsov,\" said protestor Alexander Lektman, 26. \"He went on a hunger strike and that is a great political gesture. I want our unjust political system to change.\" The demonstration, which gathered several hundred people around a statue of Russia\'s national poet Alexander Pushkin -- a prominent landmark in the very centre of the city -- would have been almost unthinkable even weeks ago. Before the wave of protests erupted after the December 4 parliamentary elections, police would habitually crack down on opposition protests, often arresting dozens of people.