* Investigators issue stern warning to protest leaders
* Critics say latest step in Kremlin crackdown on dissent
By Thomas Grove
MOSCOW, Oct 17 (Reuters) - Russian investigators began criminal proceedings against a prominent leader of protests against President Vladimir Putin on Wednesday, saying a documentary on a pro-Kremlin TV channel showed evidence Sergei Udaltsov had plotted mass disorder.
Government critics and civic rights groups says the Kremlin is carrying out a coordinated clampdown on dissent by exerting legal pressure to try to sideline activists who have led the biggest opposition protests in Putin's nearly 13-year-long rule, spurred by allegations of widespread election fraud.
Law enforcement officials raided Udaltsov's Moscow apartment around daybreak and state television showed him being taken away for questioning by a half a dozen officers in black balaclavas.
"I have committed no crime. My only crime is speaking the truth," Udaltsov told Vesti-24 as he was led away.
Police were also searching the homes of two associates facing the same charges, which carry a jail sentence of up to 10 years. Udaltsov was released but ordered not to leave Moscow.
His criminal case focused on allegations aired in a documentary on NTV television that he received money and orders from an ally of Georgian President Mikheil Saakashvili, an adversary of Moscow, to cause unrest in Russia.
A statement from investigators said they were looking at allegations that Udaltsov - a leftist known for his shaved head, leather jacket and frequent short-term jailings for disobeying police - had planned "terrorist acts" in Russia.
Udaltsov, one of the leaders of a series of opposition protests that have brought tens of thousands of people into Moscow's streets calling for "Russia without Putin", has denied the allegations aired on NTV earlier this month.
The Investigative Committee, which answers only to the president, also issued a stark warning to protest leaders who Putin has at times publicly ridiculed and sought to discredit by saying they receive Western support.
"Those who think they can with impunity organise riots, plan and prepare terrorist attacks and other acts that threaten the lives and health of Russians, you underestimate the Russian special services' professionalism," the statement said.
The Kremlin appeared to be probing how far it could go in cracking down on foes, opposition lawmaker Dmitry Gudkov said.
"They will be taking the temperature of society. The repressions will continue," Interfax quoted him as saying.
Since Putin's May 7 inauguration, he has signed laws increasing restrictions on non-government organisations and raising fines for disorder at demonstrations.
The Investigative Committee, led by Putin loyalist Alexander Bastrykin, has also pressed charges against opposition leader Alexei Navalny for allegedly organising the theft of timber from a state firm - charges he denies.
"They are not just fabricating cases, they are dreaming them up," Navalny, who also faces 10 years in prison, said of the case against Udaltsov.
The Investigative Committee said it had launched the prosecution after studying documentary footage from NTV that showed Udaltsov and others meeting with the former head of the Georgian parliament's defence committee Givi Targamadze in the Belarusian capital of Minsk.
Georgia, under Saakashvili's administration, cut diplomatic relations with Moscow after a five-day war with Russia over two Kremlin-backed breakaway regions in the South Caucasus nation.
"The voice recorded in the footage shot with a hidden camera... belongs to Udaltsov, and the meeting, excerpts of which are shown in the film, took place in the second half of June 2012," the statement said, referring to NTV documentary "Anatomy of a Protest 2".
NTV, owned by the media arm of state-controlled gas export monopoly Gazprom, has been used regularly to criticise those who have fallen foul of the Kremlin.
The protest movement grew out of allegations that serious fraud enabled Putin's United Russia party to win a parliamentary election last December despite declining popular support.
But the victory of United Russia in local and regional elections on Sunday underscored the opposition's failure so far to parlay street protests into an effective, broad popular challenge to the president.