Among the 21 officers, 16 were injured on Sunday, with one suffering massive bleeding after poled by a sharpened item in his back and five others were injured on Saturday after hit by bricks, according to a press briefing of the police.
There were also reports of a Hong Kong police officer firing a gunshot during protests on Sunday, the first time a live round was used since demonstrations broke out in June.
The police told the media that the gunshot was fired as a "warning" to protesters and added that six officers had drawn their pistols and one had fired the warning shot into the air.
Police Superintendent Leung Kwok-wing did not say where the shot was aimed but told the South China Morning Post that the shot was fired because the lives of police officers were "under threat".
Mak Chin-ho, Assistant Commissioner of Police, said protesters used deadly weapons, including bricks, metal poles, long sticks and petrol bombs, to assault police officers.
"These attacks are intentional, planned and organized," he was quoted as saying by Xinhua news agency.
"Through the whole of afternoon on Sunday, a large gang of rioters threw numerous self-made petrol bombs at police officers in Tsuen Wan," Mak said. "In certain incidents, they randomly threw petrol bombs from height at people on protest sites, hitting innocent passersby."
Besides assaulting police officers, the protesters also obstructed roads and vandalized shops and tunnel facilities, said the report.
"Their destructive acts are rampant and reckless. They are not to be tolerated by any civil society," Mak said.
During the melee, one older man was seen on his knees pleading with the police not to shoot. The police also fired rubber bullets and tear gas, and used two water cannon vehicles to clear barricades and disperse crowds, the BBC reported.
During the violent incidents on Sunday, 45 men and nine women aged 12 to 51 were arrested for offences including unlawful assembly, possession of offensive weapon and assaulting police officers. The number of the arrested protesters totalled 86 during the whole weekend.
Protests in Hong Kong were sparked in June in opposition to a now-shelved extradition bill, but have since morphed into broader anti-government protests, and developments on Sunday marked a serious escalation in the unrest.
Activists have criticized the police for being "heavy-handed" during the pro-democracy rallies, which saw several people hospitalized after violent clashes on Saturday.
( With inputs from IANS )