HONG KONG—Televised footage that appears to show police officers beating a protester early Wednesday prompted outrage in Hong Kong, putting the police and government back on the defensive just a day after they declared their first victory over the protests by opening a main road.

The video was shot in the early morning hours and shows half a dozen police officers hauling a person in plastic handcuffs around a dark corner and then taking turns kicking him repeatedly while he is on the ground. It was captured in a 51-second video that clearly shows the word “police” on the shirts of the men carrying and beating the protester. (Latest News:Police Clear Protest Site in Mong Kok)

“This is a Hong Kong we don’t recognize anymore,” said Lee Cheuk-yan, a Hong Kong legislator, at a news conference, referring to front-line police as “black cops.” Pro-democracy legislators including Mr. Lee are calling for a full investigation into the highest levels of the police department.

The confrontation has put police back on their heels after they had successfully reopened a main thoroughfare in the city Tuesday.

Protesters gathered in the tunnel late Tuesday and quickly surrounded a group of police, who were forced to retreat. The protesters then built barricades in the tunnel, stacking concrete blocks they pulled from storm sewers.