Police officer Vadim Boiko was reportedly attacked on September 15, but the incident was made public only today.
Boiko was hospitalized with a concussion and other head injuries. He told journalists who came to the hospital that an unknown man had approached him shouting: "This is your punishment, Pearl Ensign!" and hit him on the head.
Boiko said that the attack on him is most likely connected with his "behavior during the public gathering on July 31."
Boiko became known as the "Pearl Ensign" after a video of a police intervention was posted on the Internet showing a police officer, who looked like Boiko, insulting and beating demonstrators and dragging them by the hair.
The policeman in the video wore a bracelet, and Internet users started calling him the "Pearl Ensign" before he was identified as Boiko.
Police in St. Petersburg opened an investigation after the video sparked protests in Russia.
On September 1, Boiko turned himself in to police and was identified by Dmitry Semenov, who was allegedly beaten and insulted when police violently dispersed the July 31 protest by activists demanding freedom of assembly.
Boiko was told by police not to leave the city until the investigation was completed. He was charged with abuse of power.
Investigative Journalism Agency deputy head Yevgeny Vyshenkov expressed doubt about the report of Boiko's beating. He said it was very strange that Boiko's ordeal and hospitalization was made public only several days after the reported attack.
"The government should react to such sort of attacks immediately, not a week later, no matter who the victim is -- the 'Pearl Ensign' or a leading journalist -- therefore the whole story looks like a dubious attempt to transform Boiko from a power abuser into a victim," Vyshenkov said.
If found guilty of abusing his position as a police officer, Boiko could face 10 years in jail.