Police clashed with protesters early on Sunday, with the government saying 20 people were injured. It was the fourth night of violence after nearly three weeks of largely peaceful pro-democracy rallies.
Dozens of police with shields and helmets pushed into a crowd of protesters gathered at barricades in Mong Kok, striking at them repeatedly with batons. Some demonstrators had to be carried away on stretchers and others treated for head wounds, broken or cracked bones and bruising, according to reporters and medics at the scene.
Police said in a statement on Sunday that they had used "minimum force" as protesters "suddenly attempted to charge" their cordon lines.
However protesters said they had done nothing to provoke officers.
The city’s government information service said 20 people involved in protest activities had been injured between 10pm and 6am - but would not specify how many were demonstrators or police, the extent of the injuries, or if they all took place in the Mong Kok area.
Hong Kong has been rocked by rallies, which at times have drawn tens of thousands of people to three main protest sites in the city - the second-largest of which is Mongkok.
The government had confirmed earlier on Saturday that it would open talks with student leaders on Tuesday. The city’s deputy leader Carrie Lam told reporters the talks - which will be broadcast live - would be focused on constitutional reform, with both sides allowed to bring five members to the meeting.
The moderator selected for the talks is Leonard Cheng, President of Lingnan University. Students fear he will not be impartial as he was an advisor to Chief Executive Leung Chun-Ying's election campaign.
But hopes of any breakthrough are slim, with the government unlikely to cede to protesters’ core demands - the resignation of the chief executive and free leadership elections in 2017.
Beijing insists that candidates for the vote must be agreed by a committee expected to be loyal to China, and Leung has warned that the country’s communist authorities have no intention of backing down.
Increase in violence
Increased force has been deployed in the last few days as officers fight to clear blockaded roads.
Some protesters could be seen with helmets and makeshift protective gear fashioned out of household items including baby mats.
One volunteer medic said that she had seen four people with head injuries with "serious bleeding" as well as a fractured back.
"One of the men with a head injury had been hit by a police baton three times," said Carla Chau, 20, a student volunteering at the first aid station set up at the Mong Kok site.
Some injured protesters were taken to nearby Kwong Wah hospital, where activists said at least 10 or more were being treated for injuries to their legs, arms and heads, including suspected bone breaks or cracks..
One protester said police had reacted when a front line of demonstrators blocking a key road opened their umbrellas.
"They hit us without any reason when we were standing behind the roadblock. I was hit by a police stick four or five times. I protected myself with my hands and they hit my body ... there was blood all over my head and they took me for medical treatment," said Jackie, 30, sitting with his head bandaged and blood still on his T-shirt.
"We didn’t do anything - some people behind me opened out their umbrellas and then the police started hitting people. There was no aggressive action on our side."
One of the student protest leaders, Lester Shum from the Federation of Students, was at Mong Kok to show support, reports the South China Morning Post. He said that the demonstrators there were upholding the principles of civil disobedience and he didn't know why officers had charged at them.
"We are not gangsters. Even if you beat us until we bleed, we will come back as we want genuine universal suffrage and civil nomination," said Shum.
Police said protesters tried to charge police cordons by pulling aside barriers and shoving officers, confirmed three protesters and an officer were injured. They have said before that they are acting in a restrained manner in the face of protesters determined to reoccupy areas of the city.
Accusations that police are being used as a political tool have dogged the highly respected force, which has a reputation across Asia for honest and impartial policing.
But video footage emerged on Wednesday showing plainclothes officers beating a handcuffed protester as he lay on the ground, just over a fortnight after riot officers fired teargas at crowds.
Hongkonger v Hongkonger
Many people have become more and more angry over the disruption caused by the protests. Blocked roads mean heavy traffic jams and local companies complaining of a downturn in business.
The crowds of protesters have shrunk dramatically from their peak of tens of thousands earlier in the month.
Meanwhile RTHK reported that a man had been arrested in Tin Shui Wai on Saturday night on suspicion of encouraging people over the internet to take part in illegal assemblies in Admiralty and Mong Kok and to charge at police.