* Investigators say activist confessed
* Activist says he was abducted and tortured
* Kremlin critics say case is part of crackdown on dissent (Adds protest in support of Razvozzhayev, investigators comment, paragraphs 12-13)
By Thomas Grove
MOSCOW, Oct 22 (Reuters) - Russian investigators said on Monday that a man accused of plotting to foment unrest with other opponents of President Vladimir Putin had turned himself in and confessed, but a video on the Internet showed the activist saying he had been tortured.
Investigators said Leonid Razvozzhayev had voluntarily confessed on Sunday in a high-profile case that Kremlin critics say is part of a clampdown aimed at sidelining activists who have led the biggest protests of Putin's near 13-year rule.
His supporters disputed that account, saying Razvozzhayev had in fact been detained by force in the Ukrainian capital Kiev on Friday. Video posted on the Internet by the opposition lawmaker who employs him as an aide shows him accusing the authorities of torturing him.
His detention and alleged confession could have far-reaching implications for the opposition since one of the other suspects in the case is Sergei Udaltsov, a prominent protest leader who has played a major role in organising anti-Kremlin demonstrations.
They and another associate were named in a criminal investigation opened last week based on what the authorities said was evidence they had plotted disorder and violence around Russia. The alleged evidence, which was collected using a hidden camera, was broadcast on pro-Kremlin TV.
All three could be sentenced to 10 years in jail if convicted.
The federal Investigative Committee, Russia's top investigative body, said a court had ordered Razvozzhayev be held for two months.
"Razvozzhayev addressed himself to the ... Investigative Committee and said he wanted to confess," the committee, which answers directly to Putin, said on its website.
It said he had explained "in detail" how the three men and others had planned to organise "mass disorder" and also described their involvement in violence at a protest on the eve of Putin's presidential inauguration on May 7.
But a video on the blog of opposition lawmaker Ilya Ponomaryov, the opposition lawmaker for whom Razvozzhayev works as an aide, painted a different picture and showed Razvozzhayev being led from a building into a Moscow police van.
"Tell them I was tortured," he is heard saying. "They tortured me for two days. They abducted me in Ukraine."
Some 50 people, including Udaltsov, gathered outside the Investigative Committee building in Moscow on Monday evening to protest. "This is not just repression, this is terror," he said.
The Investigative Committee said a medical examination on Razvozzhayev found no injuries on his body.
Ponomaryov said on his blog that Razvozzhayev had disappeared after visiting the office of the U.N. High Commissioner for Refugees in Kiev.
An employee at the UNHCR office, Alexandra Makovskaya, said that Razvozzhayev had been there on Friday, saying he had wanted to seek political asylum, and had received advice.
Udaltsov, who denies the allegations, was released after questioning and a search of his apartment on Thursday and was ordered to remain in Moscow. His aide, Konstantin Lebedev, was charged and ordered held in custody for two months.
The investigations into Razvozzhayev, Udaltsov and Lebedev came after the pro-Kremlin NTV TV station broadcast allegations the trio had received money and orders from an ally of Georgian President Mikheil Saakashvili, an adversary of Moscow.
Since his inauguration, Putin has signed laws increasing fines for violating public order at protests and placed restrictions on foreign-funded non-government organisations, moves critics say are aimed at silencing dissent.
Another prominent leader of the street protests, Alexei Navalny, has also been ordered not to leave Moscow and faces up to 10 years in jail in a case unrelated to the one against Udaltsov, Lebedev and Razvozzhayev. (Additional reporting by Gabriela Baczynska, Writing by Steve Gutterman,; Editing by Andrew Osborn and Alison Williams)