Police detained over 100 people in Moscow and Saint Petersburg during demonstrations on December 31, 2010 [EPA]

Boris Nemtsov, a leader in the Russian opposition movement and a former first deputy prime minister, was sentenced to 15 days in prison by a Moscow court on Sunday.

Nemtsov was arrested along with two other opposition leaders for disobeying police while taking part in an unsanctioned New Year's Eve opposition rally.

"This is an absolute disgrace," Nemtsov told Moscow radio after his sentencing.

Yevgeny Chernousov, Nemtsov's lawyer, dismissed the charges as groundless, voicing the court's unfair selective process in deciding what evidence it wants to believe.

"It astonished us how easily the court used a variety of far-fetched arguments to deny the evidences of 13 people who were near Nemtsov and who saw everything [during the rally]," Chernousov said.

"At the same time it [the court] accepted the evidences of two policemen."

City authorities allowed the protesters to assemble on a small section of a central Moscow square a few blocks from the Kremlin.

But Russian reports said Nemtsov and a group of other opposition leaders tried to break through the police ranks, leading to their immediate arrest.

The rally was one of many in a series of traditional end-of-month demonstrations called by opposition leaders to assert Article 31 of Russia's constitution, which grants Russians freedom of assembly.

Moscow police detained over 100 people in Moscow and Saint Petersburg during the New Year's Eve protests.

Nemtsov's arrest came just days after Kremlin critic and former oil tycoon Mikhail Khodorkovsky's jail sentence was extended by six years.

Khodorkovsky's arrest was the first time in several years that an opposition leader had been imprisoned, a sign of the government's harsher stance against those who question the Kremlin's policies.