The accused officers were removed from their posts after the assault, which occurred early Wednesday when police swooped to clear newly erected barricades blocking a road near government headquarters in the city's Admiralty district.
Just 24 hours later a small contingent attempted to retake an underpass on the same road, leading to a confrontation that saw protesters pepper-sprayed and police heckled with chants accusing them of links with criminal triad gangs.
They are angered at China's insistence that it vet candidates standing for election as the city's next leader in 2017.
On a visit to London, Hong Kong's Secretary for Justice Rimsky Yuen said he had not seen the footage of the alleged assault but told AFP there were "very well-established procedures" in Hong Kong to handle police complaints.
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Referring to any possible criminal prosecutions arising from the alleged police beating, Yuen said: "I can guarantee that if there is any need to move in that direction, my department will handle the matter in an independent and impartial manner."
"The major thing is people have realised how police become corrupt under a government that doesn't represent the people of Hong Kong," said Kay Wong, 25, a university research assistant.