The government fears the attempt to storm the Legislative Council building on Friday night was a trial run by activists for the Occupy Central campaign of civil disobedience, a source close to the administration revealed.
Protesters used bamboo poles to try to force open doors to the Legco building when a crowd that had gathered to oppose government plans to build two new towns grew angry.
Inside, Legco's Finance Committee had been due to vote on a HK$340 million funding request for engineering works linked to the development plan in the northeastern New Territories, which will cost many villagers their homes.
The source said the mayhem offered an insight into the looming Occupy Central movement, which threatens to blockade the city's business district if the government does not come up with a satisfactory plan for electing the chief executive in 2017. That is the year when, under the Basic Law, the city may vote in a chief executive by universal suffrage for the first time.
"It is expected that Occupy Central … will be hijacked by the same group of people who will push the movement away from its original goal. These people will take even bigger steps to storm Legco when it scrutinises the reforms for 2017," the source said.
Chief Executive Leung Chun-ying accused the protesters of "further aggravation in the wake of a spate of illegal events".
"People with a clear mind will notice that those who carried out the violent acts are not the villagers affected," she said.
But Labour Party lawmaker Dr Fernando Cheung Chiu-hung said: "The protesters know clearly what they are opposing. The unjust development project matters not only to the affected villagers but all Hongkongers.
"I hope Lam will stop insulting the protesters and start listening to the people."
The people storming the Legco building were ordinary Hongkongers rather than political groups, said Napo Wong Weng-chi, an activist who claims he was beaten after being arrested as officers cleared the remaining protesters from the site early yesterday.
"I don't see why anyone would break into buildings when they join Occupy Central, which stresses peacefulness," he said.
Dr Chan Kin-man, one of Occupy Central's founders, believed there was little threat of the campaign resulting in a similar clash with police. Officers used pepper spray to force the crowd back from the doors.
"Even if a small group of people do get radical, we believe we will be able to handle most of them," he said.
A total of 21 people, aged 19 to 72, were arrested in the protest.
After viewing damage to the building, Legco President Jasper Tsang Yok-sing condemned the "very serious violent acts".
Activists claim they were punched and spat on by police after being arrested in the early hours of yesterday as police broke up their protest against development plans for the northeastern New Territories.
Five activists, including Jaco Chow Nok-hang, Leung Wing-lai and the League of Social Democrats' Napo Wong Weng-chi, said they were attacked in a police van en route from the Legislative Council building in Admiralty to Aberdeen Police Station.
"The police drew the curtain, turned the light off and started beating us about our heads," Chow said yesterday after being released on bail, bruises clearly visible on his forehead and ears. "They put handcuffs on three of us. They slapped and punched us, pulled our hair and even spat on our faces."
The violence stopped when they reached the station, where the activists were taken to a washroom to clean the blood off their faces before reporting to the duty officers, Chow said.
Wong said there about 10 police officers in the van with them. "Some of the police even kicked us," he added.
Labour Party Legislator Dr Fernando Cheung Chiu-hung also condemned the police for "using force" against his assistant, who he said had been handcuffed by officers even after he had introduced himself.
The assistant was also assaulted by several officers inside a police vehicle, Cheung said.
Police Commissioner Andy Tsang Wai-hung refused to comment on the accusations, but he said all complaints received were investigated impartially.
The Civil Human Rights Front described the alleged violence as "barbaric, illegal and unprofessional". It also urged the government watchdog to send representatives to any future rallies.
Meanwhile, the RTHK Programme Staff Union condemned the police for initially hauling away its TV reporter, Luther Ng Lap-tak, as officers moved in to clear the 100 or so people who were still protesting at the Legco site after midnight.
RTHK and the Hong Kong Journalists Association urged police to apologise over the incident.