* Treatment of Kremlin critic Navalny sparked protests

* Some protesters say they were injured by police

* Kremlin denies police repression, defends police action

By Polina Nikolskaya

MOSCOW, Feb 10 (Reuters) - Alexei Borisov was diagnosed bydoctors as having a punctured lung, three fractured ribs and abroken tooth after he attended a rally on Jan. 31 in support ofRussian opposition politician Alexei Navalny.

The 42-year-old truck driver said he was injured by policewho detained him after he led a march in central Ryazan, 200 km(120 miles) southeast of Moscow, shouting "Freedom to AlexeiNavalny" and slogans against President Vladimir Putin.

"I was lying face down on the (police car) floor... Theybegan to hit me, I didn’t even see how many of them there were,"Borisov said.

Reuters did not witness the incident Borisov described buthe provided medical documents that confirmed his injuries and aphotograph showing him lying in hospital. The documents did notshow where or how he sustained the injuries.

Asked about his case by Reuters, the interior ministry,which is in charge of the police, did not comment.

Borisov is one of more than 11,000 people who have beendetained at protests in recent weeks over Navalny's arrest andimprisonment, according to OVD-Info, a non-governmental groupthat monitors arrests during mass protests and seeks to preventhuman rights abuses and political persecution.

An OVD-info representative said the group knew of about 116cases of alleged police violence following the recent protestsand feared many more had not reported their injuries.

Reuters has been unable to determine how widespreadcomplaints of police brutality have been at protests that haveinvolved tens of thousands of people in the last few weeks.

The vast majority of protesters have not complained of beinghurt and no deaths have been reported. But reporters say policehave been much more forceful, and in some cases more violent,than at most other political protests in recent years.

The interior ministry, the Investigative Committee whichinvestigates major crimes and Russia's National Guard did notrespond to requests for comment about the police conduct.

The Kremlin has denied repression by the police. It has saidany cases of alleged police brutality are being looked into butthat there have been many more cases of riot police officersbeing attacked by protesters than vice versa.

The authorities have called the protests illegal becausethey have not received official approval to go ahead, and saidsuch rallies risk spreading COVID-19.

TASERS AND BATONS

Navalny was arrested in January after returning to Russiafrom Germany, where he was treated after being poisoned inSiberia with what many Western countries said was a nerve agent.Navalny blamed Putin for the attack but the Kremlin hasdismissed the accusations and questions whether he was poisoned.

Reuters reporters in Moscow and St Petersburg have seenprotesters tasered and beaten with truncheons, and have spokenwith eight who say police used violence against them.

A Reuters reporter in Moscow witnessed a protester fallingto the ground and screaming in pain during his arrest at aprotest on Jan. 31, and asking the police to stop tasering him.

After a week in jail, the same protester, 30-year-oldfitness trainer Soso Glonti, told Reuters a policeman hadtasered him even though he was not resisting arrest.

"I know how to endure (pain), but the fifth or sixth timethey tasered me I lost my temper. It was the fifth time when Istarted screaming," he said.

Medical records of four protesters viewed by Reutersdetailed injuries including a broken arm, concussion, a headinjury, bruises, and an eyelid wound.

The RIA news agency said the National Guard is looking intoan incident in which an officer hit cameraman FyodorKhudokormov, 18, who told Reuters he was beaten around the headwith a baton on Feb. 2 even though he was clearly identified asa journalist.

Victor Lipatov, a 49-year-old lawyer, told Reuters he washit on the head and arm with a baton while standing in front ofriot police and holding hands with other protesters.

OVD-info group says Russia has opened 36 criminal casesagainst people who attended rallies over alleged use of forceagainst riot police.

The Investigative Committee said 21 criminal cases wereopened after the first rally on Jan. 23, including againstprotesters accused of striking policemen. It has not said howmany have been opened since then.

The Investigative Committee says it has opened criminalcases against protesters who threw fireworks or other objects atpolicemen, kicked or punched them, or sprayed pepper gas.

Russia's Committee Against Torture monitoring group said ithad filed six complaints to the Investigative Committee overalleged use of violence by riot police but had not yet receivedany answers. They included Lipatov's case.

No criminal cases have been announced against policeofficers or National Guardsmen.(Reporting by Polina Nikolskaya, Editing by Andrew Osborn andTimothy Heritage)