Dozens of riot police in Hong Kong pushed into a crowd of protestors at the barricades in Mongkok, repeatedly striking them with batons.
Some demonstrators had to be carried away on stretchers while others were treated for head wounds, fractures and bruising, according to AFP journalists at the scene.
In a statement issued on Sunday, police said they used "minimum force" as the crowed "suddenly attempted to charge" their cordon lines. But protestors deny provoking officers.
An eyewitness 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," Carla Chau, a medical student volunteering at a first aid station set up at the Mongkok site, told Channel News Asia. Some injured protesters were taken to nearby Kwong Wah hospital.
One protester said that 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."
Hong Kong's government confirmed that it would open talks with student leaders on Tuesday.
The city's deputy leader Carrie Lam told reporters the talks – to be broadcast live – will focus on constitutional reform, with both sides allowed to bring five members to the meeting.
In a separate incident, a man was arrested for allegedly posting messages online that encouraged people to gather and agitate at a protest site, according to police.
The move – one of the first such arrests during three weeks of demonstrations here – could be an attempt by authorities to stop the use of the Internet and social media to mobilise large crowds.
The Hong Kong Police Force said on its website that a 23-year-old man was arrested on Saturday accused of "accessing a computer with illegal or dishonest intentions" and illegal assembly.
A police investigation stated that the man had urged people to go to Mong Kok to "join an illegal assembly, attack police and paralyze subway lines," the statement said.