Police in riot gear rained blows on protesters in Mongkok on Sunday morning.
Some demonstrators were taken away on stretchers and others treated for head wounds, fractures and bruising, according to medics.
Accusations that the Chinese territory's police are being used as a political tool have dogged the highly respected force, which has a reputation across Asia for impartial policing.
Pro-democracy protesters clash with police on a street in Mong Kok on October 19, 2014 in Hong Kong.
Police said in a statement they had used "minimum force" as protesters approached their cordon lines.
Talks planned for Tuesday
The Hong Kong government, led by Leung Chun-ying, confirmed on Saturday it would open talks with students on Tuesday, speaking before the fourth night of violent clashes.
"Right now we are planning that it will take place on the afternoon of Tuesday, October 21," Leung deputy Carrie Lam told reporters on Saturday.
Lam said the talks, to be broadcast live, would focus on constitutional reform, with both sides allowed to put up five members.
But hopes of any breakthrough are slim, with the government unlikely to cede to protesters' core demands of the resignation of embattled Leung and free leadership elections in 2017.
Beijing insists that candidates must be vetted by a committee expected to be loyal to China.
Sunday's violence continued a trend of increased force being deployed in the last few days as officers attempt to clear roads at three sites.
One witness said she had seen four people with head injuries with "serious bleeding", and another with 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 medical student volunteer.
At Kwung Wah hospital activists said at least 10 or more were being treated for injuries, including suspected fractures.
One protester said that police had reacted when a front line of demonstrators opened their umbrellas and rested them on the barricades.
"They hit us without any reason when we were standing behind the roadblock. I was hit by a police stick four or five times," said the protester.
A pro-democracy protester who gave his name as Jackie waits at a hospital to be treated for a head injury sustained during clashes with police in the Mong Kok district of Hong Kong early on October 19, 2014
Police said in a statement on Sunday they had taken "resolute action by applying minimum force to disperse protesters to prevent the situation from deteriorating".
Many residents are increasingly frustrated over the disruption, as road blockages cause traffic jams in the city of seven million, and local businesses complain of a downturn.
The protest crowds have shrunk from their peak of tens of thousands earlier in the month.
The mass rallies are one of the biggest challenges to Beijing's authority since the Tiananmen Square pro-democracy protests of 1989.
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