Human rights organization Amnesty International has called for the immediate release of several Russian opposition leaders and declared them prisoners of conscience.
In a statement posted on its website it said "the strangling of the freedom of assembly in the Russian Federation must stop."
The Russian authorities' "failure to protect the right to freedom of assembly" has once again become apparent during the gathering of peaceful demonstrators in Triumfalnaya Square, in central Moscow, on December 31, 2010 and "in the course of events that followed," it said.
Russian opposition leader Boris Nemtsov on Saturday expressed his thanks to everyone who has taken to the Moscow streets to protest his 15-day imprisonment following a sanctioned rally on New Year's Eve.
The co-chair of the Solidarity movement remains in a detention center in Moscow.
Activists have protested every day against the jailing of Nemtsov and three other opposition leaders. Ilya Yashin has been released after serving his five-day sentence but Konstantin Kosyakin and Eduard Limonov are serving 10- and 15-day sentences, respectively.
Amnesty International said it believes that "these people have been detained solely for peacefully exercising their right to freedom of assembly and freedom of expression," which is in violation of Russia's international obligations and Constitution, and "considers them to be prisoners of conscience."
"Those still in detention must be released immediately," it said.
MOSCOW, January 8 (RIA Novosti)
