By Gabriela Baczynska and Nastassia Astrasheuskaya

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 and violence across Russia.

A Moscow court said Konstantin Lebedev would remain in custody until at least December 16. His boss Sergei Udaltsov, known for his shaved head and black leather jacket, was released on condition he remain in Moscow.

The charges came after a pro-Kremlin television channel aired a video filmed by a hidden camera showing what it claimed were Udaltsov and his aide discussing the organization of mass disorder with a man identified as a Georgian lawmaker.

Udaltsov and Lebedev, who faces 10 years in jail if convicted, were taken in for questioning 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. Both men deny the accusations leveled against them.

Their allies said on Thursday they believed letting Udaltsov go and keeping Lebedev in detention was aimed at mounting pressure on the latter to get an admission of guilt from him that would then serve in a court case.

UNDER PRESSURE

"We think this is a classic arrangement by law enforcement agencies," said Ilya Ponomaryov, one of the founders of Udaltsov's Left Front and now an opposition lawmaker in the State Duma.

"Most likely it's aimed at showing to Konstantin, a person much-less recognized, that he is just a simple guy. We will not touch the famous guy but will turn you into a prison trash. So give the right testimony," he told a press conference.

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

The Kremlin has in the past tried to ridicule Putin's critics as well as discredit them in the eyes of regular Russians by presenting them as foreign-funded provocateurs or hooligans.

"There are no real grounds for this, this is pure propaganda to open a criminal case and then refer to him by saying he is a criminal," Ponomaryov also said of Udaltsov' charges.

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.

Other prominent Putin critics have also been presented with criminal charges or have investigations opened into them.

(Editing by Myra MacDonald)