Hong Kong police are facing accusations of failing to protect pro-democracy activists who were attacked by unidentified assailants on Sunday.
Opposition lawmakers say police inaction allowed criminals to beat up peaceful demonstrators after a rally.
Dozens were injured when they were set upon by masked men wielding sticks.
Police say their forces were stretched during another day of unrest. Hong Kong has been rocked by weeks of protests sparked by an extradition bill.
Late on Sunday video footage showed groups of men - dressed in white shirts and suspected to be triad gangsters - beating passengers with rods at a train station in the Yuen Long area.
There is speculation that the attackers have links with criminal gangs, known as triads. Forty-five people were injured, with one person in critical condition.
Snippet of a live broadcast from lawmaker Lam Cheuk ting, showing self-professed pro-Gov't mobsters attacking passengers in train cars at #MTR #YuenLong Stn. #HongKong has 1 of the world's highest cop to population ratio. Where were @hkpoliceforce? Lam was injured as shown live. pic.twitter.com/Aq5JmJlf5u— Ray Chan (@ray_slowbeat) July 21, 2019
Hong Kong's Chief Executive Carrie Lam described the attacks as "shocking". She also condemned protesters for defacing China's main representative office in the city earlier in the day.
How have activists responded?
One of the pro-democracy lawmakers injured in the attack, Lam Cheuk-ting, criticised the police response and suggested the assailants were linked to gangs.
"Is Hong Kong now allowing triads to do what they want, beating up people on the street with weapons?" he told reporters.
Another pro-democracy legislator, Ray Chan, tweeted: "Hong Kong has 1 of the world's highest cop to population ratio. Where were @hkpoliceforce?"
Alvin Yeung, who leads the Civic Party, said: "This is triad gangs beating up Hong Kong people. Yet you pretend nothing had happened?"
Hong Kong police chief Stephen Lo defended his forces, saying his officers were busy dealing with violent anti-government protests elsewhere.
"Our manpower is stretched," he said. Mr Lo called suggestions that police had colluded with triads a "smear", adding his officers would pursue the attackers.
Late on Monday night police said six men had been arrested for unlawful assembly in Yuen Long.
Pro-democracy protesters were attacked as they travelled back from a rally in the centre of Hong Kong, where riot police had fired tear gas and rubber bullets at protesters.
The masked men stormed Yuen Long MTR station at about 22:30 local time (14:30 GMT).
Local media said they were targeting people dressed in black - the colour most protesters were wearing.
One journalist, Gwyneth Ho, was attacked while she was in the middle of live streaming for news website Stand News. She is currently in hospital.
This is a clip from Gwyneth Ho (@StandNewsHK)'s live video. Look at how vehement and vicious the white-shirts are in their attacks on protesters. Look at their matching weapons – their long wooden sticks & umbrellas. #antielab #YuenLong— Jun Pang (@hyjpang) July 21, 2019
One witness - Galileo Cheng, 34 - told the BBC that he had suffered several blows to his back and arms when he stepped in to try to help her.
The Hong Kong Journalist Association said some reporters on the scene had equipment seized.
Police arrived at the station after 23:00, by which time most assailants had left.
What led up to this?
Mass protests have been held for weeks, initially over an extradition deal with mainland China, which the Hong Kong government has since suspended.
Critics said it would undermine the territory's judicial independence and could be used to target those who spoke out against the Chinese government.
The unrest has now spread to cover broader demands for democratic reform and reflect concerns that freedoms are being eroded.
Sunday's rally drew more than 430,000 people, but police put the figure at 138,000.
In a rare act, protesters defaced the liaison office, China's central government building. One of the graffiti slogans read: "You taught us peaceful marches are useless."
Ms Lam strongly condemned the vandalism by "radical demonstrators", saying they had acted "maliciously" and "challenged the nation's sovereignty".
On Saturday a counter-rally, in support of the police and against protest violence, drew 300,000 people according to organisers and 103,000 people according to police.
Hong Kong is part of China but run under a "one country, two systems" arrangement that guarantees it a level of autonomy. It has its own judiciary, and a legal system that is independent from mainland China.