Video report by ITV News Correspondent Nick Wallis
Police in Hong Kong have fired tear gas into a train station in a bid to force protesters out.
Video showed police officers firing the canisters into a station in Kwai Fong, filling it with smoke, but it was not clear how many protesters were inside the station at the time.
Although tear gas has been widely used against the demonstrators so far, it is rarely used in enclosed spaces.
As protests enter their tenth week, police fired tear gas at protesters as they clashed with authorities.
A number of people were said to have been injured on Sunday, including a police officer who is thought to have been burnt by a petrol bomb thrown by a protester.
On Monday, police in Hong Kong showed off water canons which could be used against protesters.
Initially protests in the former British colony began as a campaign against a controversial extradition bill which would have allowed suspects to be sent to the mainland.
Since then, Hong Kong's chief executive Carrie Lam has said that the bill is "dead", but the demonstrations have morphed to include Ms Lam's resignation, democratic elections, the release of those arrested in earlier protests and an investigation into police use of force against the protesters.
No end appears to be in sight for the protests, with neither side conceding.
On Sunday, authorities attempted to clear protesters from two parts of the Asian financial capital, as they blocked traffic ahead of another night of showdowns with riot police.
Protesters hurled bricks at officers and ignored warnings to leave the Sham Shui Po area before tear gas was deployed, police said, calling the march an "unauthorised assembly".
Nearby, protesters wearing gas masks gathered outside a police station in Cheung Sha Wan, as officers wearing their own protective gear looked down at them from a tall wall around the station.
Tear gas was also deployed in central Hong Kong on both sides of Victoria Harbour, in the Tsim Sha Tsui area on the Kowloon side and in Wan Chai on Hong Kong Island.
At one point protesters blocked the entrance to a plaza to prevent police from entering.
At Tsim Sha Tsui police station, authorities said one officer was taken to the hospital after he suffered burns on his legs from a petrol bomb thrown by a protester.
Across Victoria Harbour, a large group of mostly young protesters marched down the middle of Hennessey Road, a main shopping street in the Causeway Bay area, as a rally was held in nearby Victoria Park.
Many protesters wore face masks to shield their identities, and a few had helmets on.
Others just carried backpacks and wore the black T-shirts that have become their uniform.
"We hope the world knows that Hong Kong is not the Hong Kong it used to be," said one protester, Louisa Ho.
"China is doing more and more to pressure Hong Kong, its people and its organisations."
Banners at the rally in Victoria Park read "Give Hong Kong back to us" and "Withdraw the evil law," the latter a reference to an extradition bill that was the original spark for the protests.
A large crowd sat under umbrellas, which are both a protest symbol in Hong Kong and protection from the summer heat.
Hannah Yu, an organiser, said the protest would provide a platform for people to rally peacefully.
In what has become an established pattern, groups of protesters have taken over streets or besieged government buildings after largely peaceful marches and rallies earlier in the day.
"There will still be citizens going out on the streets to protest, but we cannot control them and we do not have the authority to control them," Ms Yu said.
Police permitted the rally in Victoria Park but denied a request by organisers to also have a march in the eastern part of Hong Kong Island.
Police also denied permission for the march in Kowloon, but protesters went ahead anyway.
In the North Point neighbourhood, a former Communist stronghold, brawls broke out throughout the day among a group of unidentified men, journalists and protesters.
The men, some carrying Chinese flags, surrounded journalists in neon press vests and yelled at them.
Dozens of police stood guard in an attempt to control the situation, which repeatedly descended into scuffles.
Newly pasted red banners around the neighbourhood declared: "Protect Hong Kong; Fujianese people rise up to defend the homeland."
People from China's southern Fujian province rallied in support of police on Saturday.
Officers in North Point on Sunday questioned four men dressed in white shirts - the uniform of thugs who injured 44 civilians inside a railway station last month.
It was not clear if the four men were arrested.
North Point was also the site last week of a fight between wooden pole-wielding men and protesters who defended themselves using makeshift shields and traffic cones.
Tiffany Law, a resident of one of the neighbourhoods that had clashes, said she came out to help protesters after hearing that many were arrested earlier in the night.
She said many area residents did not support the demonstrations.
"But if they see how the police throw all this tear gas, they might understand," she said.