As violent clashes erupted between protesters and the authorities late Wednesday afternoon local time, Hong Kong Police Commissioner Steven Lo Wai-chung said the demonstration was being considered a "riot."
Rubber bullets, pepper spray and hand-thrown tear gas were used to push back protesters who had occupied the city's main thoroughfare near the government headquarters, as well as the roads around it, Lo confirmed.
He added that police had been left with "no choice but to start to use force."
Ambulances were seen rushing towards Harcourt Road, the main site of Wednesday's protest, amid reports of people being removed from the scene on stretchers. Lo said the number of officers injured was still being tallied.
The city's legislative council had been due to hold the second reading of the controversial bill Wednesday morning local time.
Up to 5,000 police in riot gear were deployed to guard the building. Protesters were seen wearing helmets, goggles and heavy-duty workman's gloves, and pulling bricks from the sidewalks. Hundreds of businesses, parents and teachers called for a boycott of works and school on Wednesday to show their opposition to the bill.
By Wednesday morning, tens of thousands of mainly young people had arrived in the area, blocking streets and bringing central Hong Kong to a standstill.
In a pre-recorded address on Wednesday, Hong Kong's leader Carrie Lam condemned the "planned and intentional riots" and accused protestors of attempting to cross the police's line of defense.
"This has harmed the safety of normal citizens, teenagers who planned to express their views peacefully," she added.
On Monday, she said safeguards had been added to the bill to protect human rights and had received no instruction from Beijing to push it forward. Hong Kong's lawmakers had planned to dedicate 66 hours across five days to debating the bill.
"Hong Kong people are furious," senior Democratic Party lawmaker James To said Tuesday. "Our chief executive just ignored the people's voice, despite the peaceful rally of a million Hong Kong people."
Sunny Chan, an 18-year-old protestor on the streets Wednesday, said she was "angry" that the government failed to pay attention to Sunday's protests. "We choose to come out today and stand in the front and protest and try to protect my freedom," she said.
Protestor Marco Leung, 23, said there would be no difference between Hong Kong and China if the law was passed. "We are not China," Leung said. "Police should protect the citizens, not the government."
CNN's Ben Westcott and James Griffiths contributed to this report from Hong Kong.