At police headquarters in the nearby district of Wan Chai, hundreds of people gathered outside into the early hours of the morning to express outrage at the beating, with dozens queuing to lodge formal complaints over the incident.
Authorities on Wednesday said police involved in the beating of Ken Tsang Kin-chiu, a member of the pro-democracy Civic Party, would be suspended.
Footage of the beating has gone viral and injected fresh momentum into a protest movement that had been flagging after nearly three weeks of demonstrations over Chinese restrictions on how Hong Kong will choose its next leader.
In the latest confrontation, Hong Kong's public broadcaster RTHK said protesters rushed onto Lung Wo road next to the office of Chief Executive Leung Chun-ying, dragging plastic barriers and other objects with them. A Reuters photographer saw protesters later scuffling with a small group of police on the side of the road. Police repelled them using pepper spray.
Protesters caught police by surprise 24 hours earlier, erecting makeshift barricades to block the thoroughfare, prompting police to move in and arrest 45 people and clear the road. Traffic chaos at other protest sites in the city has severely disrupted some parts of the Asian financial hub.
At its peak,100,000 protesters had been on the streets. Those numbers have dwindled significantly. But a hardcore group of demonstrators, mostly students, has kept up the pressure on Hong Kong authorities, who have called the protests illegal.
China rules Hong Kong under a "one country, two systems" formula that accords the city a degree of autonomy and freedom not enjoyed in mainland China, with universal suffrage an eventual goal.
But Beijing has said only candidates screened by a nominating committee will be able to contest a full city-wide vote to choose the next chief executive in 2017.
"POLICE HAVE BETRAYED US"
Tsang was taken to hospital after being beaten and activists released photographs showing bruising on his face and body.
"I'm afraid that one day I will come out for a protest and the police will do the same thing to me," said Jen Lau, 28, a social worker who was sitting on the sidewalk outside the police headquarters in Wan Chai just after midnight.
"Even though he is a protester he should not be treated this way. I think the police should receive disciplinary action or something even more serious."
Several social workers printed a "wanted" poster with color pictures of the police officers they said were responsible for the beating, and were carrying it up and down the line of those waiting to file complaints.
"I think the police have betrayed us Hong Kong citizens," said Tony Yip, 23, a research assistant at a science museum. "They are using violence against ordinary citizens."
Leung has said there was "zero chance" China's leaders would give in to protesters' demands, a view shared by most political analysts and many Hong Kong citizens. Protesters have also demanded Leung resign.
China's ruling Communist Party believes it has offered enough concessions to Hong Kong in the past, and would give no ground because it wants to avoid setting a precedent for reform on the mainland, sources told Reuters.
On Wednesday, Hong Kong's most prominent tycoon, Li Ka-shing, urged the protesters to go home.
Li, Asia's richest man and chairman of property developer Cheung Kong (Holdings) Ltd (0001.HK), had made no public comment on the protests but broke his silence to say if Hong Kong's rule of law broke down it would be the city's "greatest sorrow".
"I urge everyone not to be agitated. I urge everyone not to let today's passion become the regret for tomorrow. I earnestly request everyone to return to their families," Li said.