Investigations are underway in Hong Kong after a group of men, dressed in white and armed with metal poles and bats, stormed a train station and attacked commuters.
Footage from the incident, which was broadcast live on Facebook, shows passengers screaming as they are repeatedly assaulted by the mob, who are suspected triad gangsters.
Dozens were wounded, including one critically, in what was a violent escalation in the political crisis engulfing the financial hub.
The attack unfolded after several thousand pro-democracy activists descended on China's representative office in the city, clashing with police.
Some of the victims had come from the demonstration - the latest in a series of protests over the past two months, which have seen millions take to the streets in a show of force against the Hong Kong's pro-Beijing leader, Carrie Lam.
Lam Cheuk-tin, from Hong Kong's Democratic Party was among those injured in the attack.
He said his party is now investigating.
Man-Kei Tam, the director of Amnesty International Hong Kong, told SBS News the lax response of police allowed the situation to spiral out of control.
"Police were very slow in responding to the situation," he said.
"We did not see police there in a timely way to control the situation."
Protesters hold placards reads "Form an independent investigation on legislative" as they take part in a march in Hong Kong. Source: AAP
Mr Tam believes the attack was coordinated and deliberate, but said the men were indiscriminate in who they targeted.
"They caused harm not only to protesters, but to ordinary citizens, including pregnant women and the elderly."
In a statement on Monday, police said they "strongly condemned" both violent incidents and investigations are underway.
