Police were called on to investigate whether officers had committed any criminal offences after six opponents of new towns in the northeastern New Territories said police beat them in the back of a van early on Saturday.
The activists are threatening to take legal action on top of lodging a complaint with the police watchdog.
The six were rounded up after midnight on Saturday as officers moved in to end a protest at the Legislative Council complex that had begun on Friday as the Finance Committee met to consider the government's request for HK$340 million of preliminary funds for the new-town projects.
The men arrested included two lawmakers' assistants. The six claim that while being taken by police van to Aberdeen police station, anti-triad officers turned off the lights and drew the curtains, then beat and spat on them non-stop for 20 minutes.
"I was really scared," Chow Chun-yu, assistant to Labour Party lawmaker Dr Fernando Cheung Chiu-hung, said at a media conference yesterday.
"I never imagined I would get beaten in a confined space by the so-called police … I was not sure if I would survive to tell the outside world the truth."
Once at Aberdeen station, officers allegedly instructed the activists to wash away the blood from their faces.
The six are seeking legal advice and collecting evidence in the hope of taking the matter to the courts.
The Civil Human Rights Front, organiser of the annual July 1 march, said the allegation that police turned off the lights before the alleged assault suggested "an intentional and planned abuse that most thought existed only in movies".
"Inappropriate use of force can be a criminal offence," member Icarus Wong Ho-yin said.
"We demand that the police investigate whether the officers involved have abused their powers, and make public the findings. If any criminal element is involved, we demand the police take action without delay."
He called on the Independent Police Complaints Council to send observers to protests, as it had done for the July 1 rally, to prevent police abuse of power.
Meanwhile, activist Chu Hoi-dick quoted Legco president Jasper Tsang Yok-sing as telling villagers and activists not to visit Legco this Friday so as to avoid more clashes when the Finance Committee discussion resumes. Chu said Tsang made the comment when they ran into each other at Admiralty MTR station yesterday.
Tsang, when asked by the South China Morning Post, said he had only told them to refrain from violence instead of asking them not to go at all.
He said some pro-establishment lawmakers had advised the government to postpone voting on the requested funds.