* Opposition plans first big protests for three months
* Protest leaders say Putin cracking down on dissent
* Anger over treatment of 17 over May 6 protest
By Maria Tsvetkova
MOSCOW, Sept 13 (Reuters) - Denis Lutskevich has been held in detention since Russian police dragged him away, bleeding and shirtless, at a protest against Vladimir Putin that ended in clashes four months ago.
The 20-year-old former marine says he did nothing wrong as baton-wielding riot police battled protesters throwing objects on Moscow's Bolotnaya Square on May 6, the eve of Putin's inauguration as president for the next six years.
But he has unwittingly become one of a group of 17 people held up by opposition leaders as a symbol of resistance and victims of a Kremlin crackdown as they prepare for Moscow's first big protest against the president for three months.
"Not only did they beat him but then they carted him off," said Stella Anton, Lutskevich's Moldovan-born mother, said of her son, who denies hitting the police and says he was beaten up by officers.
"He keeps recounting how he helped other people there - a girl, an old guy, a non-Russian lad in the metro. I even told him off for doing that," she told Reuters.
The opposition expects 50,000 people to march down a central Moscow boulevard near the Kremlin on Saturday to urge Putin to resign, resuming protests that began after a disputed parliamentary election won by his party last December.
They will denounce what they see as a growing Kremlin crackdown on the opposition and mounting pressure on protest organisers such as Gennady Gudkov, who is likely to be voted out of parliament on Friday and may be tried on charges of illegally running a business while in the legislature.
Anger among Moscow's new middle class, which makes up the majority of the protesters, has also grown over the treatment of the "Bolotnaya 17" - nicknamed after the square where they were protesting when arrested.
They are currently awaiting trial on charges of taking part in "mass disorders". Twelve are held in pre-trial detention centres, four are free but barred from leaving the country, and one is under house arrest. Some could face 10 years in jail.
About 400 people were detained and dozens were hurt when the riot police and protesters clashed on May 6 on the square across the Moscow River from the Kremlin.
The police say the protesters turned on them with flagpoles and metal crowd barriers, and that some tore up asphalt and threw it at them. They said more than 40 officers were hurt.
But protesters say officers wearing flak jackets and helmets struck them with batons and pushed many of them into a confined area, an action the protesters saw as a provocation.
Most of the Bolotnaya 17, almost all of whom are aged under 30, deny the charges against them, although one has pleaded guilty in exchange for leniency, his lawyer said.
Police video footage of Lutskevich shows him at the protest with his shirt ripped off and bloody marks all over his back. Oleg Arkhipenkov, 27, says he was arrested in Revolution Square - on the other side of the Moscow River - but still faces trial.
Eighteen-year-old Alexandra Dukhanina, who is under house arrest and barred from contact with the outside world, is seen in video footage wandering about the square during the protest and then throwing a dark object towards police, her lawyer says.
He is astonished by the severity of the charges against her - up to eight years in jail - and says it is not clear what the object was or whether it hit anyone.
Echoing his concerns over the possible punishment, human rights ombudsman Vladimir Lukin said: "My impression is that there were no mass violations there."
"But there were serious individual clashes and there were violations - by some citizens who threw bits of asphalt and by the law enforcement agencies who did not give people enough time to get out of this difficult situation," he said.
But Kremlin spokesman Dmitry Peskov has taken a tough line over the clashes. He was quoted by an opposition parliamentarian as saying "an injured riot police officer should be avenged by smashing protesters' livers on the asphalt."
Police trade union leader Mikhail Pashkin also defended the police action and denied accusations by the opposition that officers had deliberately provoked the protesters.
"What would the goal of this be? I can only guess," he said.
For the opposition, the treatment of the Bolotnaya 17 is one of the clearest signs that the Kremlin has toughened its stance and intends to douse the biggest protests against Putin since he first rose to power in 2000.
Human rights groups' concerns that the 17 will be given long jail sentences have grown since three members of the all-women punk band Pussy Riot were jailed for two years last month after storming into a Russian Orthodox cathedral and belting out a profanity-laced song against Putin's close ties with the church.
Opposition leaders say the crackdown also includes new laws increasing fines for protesters and controls on foreign-funded lobby groups, raids on homes of protest leaders and the charging of one protest organiser, Alexei Navalny, with embezzlement.
"Since May, we have been seeing less and less dialogue and openness on the side of the authorities, and rather more intolerance of any expression of dissenting views," European Union foreign policy chief Catherine Ashton said this week.
"Instead of stronger safeguards for the exercise of fundamental rights and freedoms, we have seen a string of measures all chipping away at them."
Saturday's march will again test the opposition's ability to attract large crowds, nine months after protests began. At their peak, witnesses say they attracted 100,000 people in Moscow. About 50,000 were at the last big rally on June 12, they said.
The opposition has struggled to unite, has not identified anyone to lead it and has gradually lost momentum since Putin won a presidential election in March with almost two-thirds of the vote and began a six-year term two months later.
The protest movement has also largely failed to take off in the provinces, where Putin's support has long been high.
Putin, who is 60 next month, has shown little concern in recent weeks and made clear he was not about to change tactics against his opponents when asked in an interview last week whether there had been a "tightening of the screws".
"What is 'tightening the screws'?" he said. "If this means the demand that everyone, including representatives of the opposition, obey the law, then yes, this demand will be consistently implemented."