Dozens of pro-establishment legislators joined the administration on Monday evening in condemning violent action throughout the day by extradition bill protesters, including an attack on the Legislative Council complex that was continuing after several hours.

"Protesters who resort to violence must stop their acts immediately. The police will take appropriate enforcement action to protect public order and safety," a government spokesman said in a statement.

Meanwhile, 42 pro-government legislators issued a joint statement of their own, rebuking the "extremely radical and violent protesters" who they said had initiated a string of illegal action.

The statement said the protesters had disturbed public order and challenged Hong Kong's rule of law.

They urged their pro-democracy rivals in the council to join in the condemnation of the demonstrators and said all "peace-loving people" in Hong Kong should distance themselves from the protest action "if they believe in Hong Kong's core values".

Meanwhile, Legco president Andrew Leung issued his second statement of the day regarding the violence at Legco.

He said he was "heartbroken" to see people damaging the complex.

In the morning, the police had said that 13 officers required hospital treatment after clashing with protesters in Wan Chai ahead of the July 1 flag-raising ceremony. The force said officers had been splashed with an unknown liquid, leaving some with breathing difficulties.

A number of protesters were also believed to have been injured in the clashes.