Hong Kong police have violently tackled suspected protesters after thousands marched in the city in defiance of a ban.
Images show police hitting people with batons and using pepper spray on a train in Hong Kong's underground metro.
Police said they were called to the scene amid violence against citizens by "radical protesters".
However it is unclear if all those injured and arrested in the metro system were involved in demonstrations.
Thousands of people took to the streets on Saturday to mark the fifth anniversary of China's government banning full democratic elections in Hong Kong.
Protesters lit fires, threw petrol bombs and attacked the parliament building.
In response, police used tear gas, rubber bullets and water cannon to disperse crowds, and fired live warning shots as they tried to clear the streets.
The latest protests came just a day after the arrest of several key pro-democracy activists and lawmakers in China's special administrative region.
Hong Kong has now seen 13 successive weeks of demonstrations.
The movement grew out of rallies against a controversial extradition bill - now suspended - which would have allowed criminal suspects to be sent to mainland China for trial.
It has since become a broader pro-democracy movement in which clashes have grown more violent.
What happened in Hong Kong's metro?
During protests, crowds gathered by Prince Edward and Mong Kok stations in Hong Kong's Kowloon neighbourhood.
Police said in a tweet that they responded at both sites after reports of "radical protesters" assaulting citizens and damaging property.
In a statement, Hong Kong's government also said some protesters had "committed arson and "hurled miscellaneous objects and iron railings" onto railway tracks, "completely disregarding the safety of other passengers".
Police spokesperson Yolanda Yu told reporters that 40 people were subsequently arrested for unlawful assembly, criminal damage and the assault of police officers.
But several people have complained about the excessive force used by authorities.
"The train stopped. Police boarded and hit me twice with a baton," an unnamed man told the South China Morning Post newspaper.
"They didn't arrest me. They were just venting their anger by hitting me," he added.
MTR, which operates the city's metro line, told local media that three stations - Prince Edward, Mongkok and Kowloon Bay - had been closed as a result of the incident. It is unclear when they will reopen.
What else happened on Saturday?
Protesters took to the streets in the Wan Chai district, many joining a Christian march, while others demonstrated in the Causeway Bay shopping district in the pouring rain. Many carried umbrellas and wore face masks.
Demonstrators - chanting "stand with Hong Kong" and "fight for freedom" - gathered outside government offices, the local headquarters of China's People's Liberation Army and the city's parliament, known as the Legislative Council.
In the Admiralty district, some protesters threw fire bombs towards officers. Earlier, protesters had marched near the official residence of embattled leader Carrie Lam, who is the focal point of much of the anger.
The riot police had erected barriers around key buildings, and fired tear gas and jets of blue-dyed water from water cannon. The coloured liquid is traditionally used to make it easier for police to identify protesters.
The police later confirmed that two officers fired into the air during operations to clear protesters from the streets. Both officers fired one shot each when they felt their lives were threatened, the police department said.
Eric, a 22-year-old student, told Reuters news agency: "Telling us not to protest is like telling us not to breathe. I feel it's my duty to fight for democracy. Maybe we win, maybe we lose, but we fight."
The recent demonstrations have been characterised as leaderless.
On Friday police had appealed to members of the public to cut ties with "violent protesters" and had warned people not to take part in the banned march.