MOSCOW (Reuters) - An aide to a prominent protest leader who organized street demonstrations against President Vladimir Putin was charged on Thursday with plotting mass disorder in Moscow.

Kostantin Lebedev was taken in for questioning, along with his boss Sergei Udaltsov, after dawn raids on Wednesday that critics say are part of a coordinated campaign to use the judicial system to sideline anti-Kremlin activists.

"Lebedev did not admit his guilt and refused to testify about the circumstances of his alleged organization of mass disorder," Russia's Investigative Committee said in a statement.

A Moscow court was expected to decide later in the day whether Lebedev, who faces 10 years in prison if convicted, would remain in custody pending his trial.

Udaltsov was released without charge on condition he remain in Moscow. Both men deny the accusations leveled against them.

The case against the two men has added political overtones because it is being conducted by the Investigative Committee, which reports directly to Putin.

The charges came after a pro-Kremlin television channel aired allegations that Udaltsov received money and orders to cause unrest in Russia from an ally of Georgian President Mikheil Saakashvili.

Saakashvili and Putin have traded barbs in public, highlighting the bad blood between the two leaders since Tbilisi and Moscow fought a five day war over two Georgian breakaway regions and broke off diplomatic relations.

Putin's critics say his return to the Kremlin in May has ushered in a new era of repression targeting those who have challenged his nearly 13 years in power.

Udaltsov helped organize a series of mass protests after allegations that serious fraud enabled Putin's United Russia party to win a parliamentary election last December despite declining popular support.

Since Putin's May 7 inauguration to another term as president - he was prime minister for the past four years - he has signed laws increasing restrictions on non-government organizations and raising fines for disorder at demonstrations.

The Investigative Committee, led by Putin loyalist Alexander Bastrykin, has also pressed charges against another opposition leader, Alexei Navalny, who denies organizing the theft of timber from a state firm.

(Reporting By Nastassia Astrasheuskaya; Editing by Jon Boyle)