The shaky footage shows a group of Russian OMON riot police hauling protesters away.
Two officers are seen dragging a protester -- apparently a woman -- by the arm. A third officer, dressed in blue camouflage and menacingly waving his baton at the crowd of protesters, suddenly turns toward the woman lying at his feet and kicks her hard in the lower abdomen, to the shocked reaction of onlookers.
The incident, which is believed to have taken place on May 6 during mass opposition rallies in Moscow, has caused deep outrage, fueled by speculation that the unidentified woman was pregnant.
The Internet in Russia was flooded with calls for the officer, widely dubbed the "May Maniac," to face justice.
Russia's Communist Party posted the video on its website and its parliamentary deputies called for an investigation.
Investigators today launched a probe into the incident and the Interior Ministry says it is trying to identify the police officer.
The video is one of several to emerge from the police crackdown on the ongoing wave of protests
sparked by the reelection of Vladimir Putin to a third presidential term.
More than 400 people were detained on May 6 alone, including opposition leaders Aleksei Navalny, Sergei Udaltsov, and Boris Nemtsov. Navalny and Udaltsov face up to 10 years in jail if convicted of causing bodily harm to police officers.
The woman's beating, together with other acts of police violence caught on camera, have fueled mounting anger at Russian authorities.
One of the videos shows a young man lying unconscious after a police officer hits him in the throat with his fist, also during the May 6 protests.
Again, the man's identity and condition are unknown.
Footage was also released on YouTube showing OMON police spraying tear gas into the faces of detained protesters.
Many Russians were further incensed when Putin's spokesman, Dmitry Peskov, said police had been too soft on protesters.
According to opposition leader and State Duma deputy Ilya Ponomaryov, he also stated that protesters who hurt riot police should "have their livers smeared over the asphalt."
Peskov confirmed his words in a subsequent interview.
-- Claire Bigg