Video report by ITV News Asia Correspondent Debi Edward
Gangs armed with umbrellas and long sticks could be seen to beat people in the station and inside a train carriage, with many commuters cowering and bleeding as they are repeatedly hit.
Commuters filmed by Stand News and iCABLE angrily accused police officers of not intervening in the attack.
The Hong Kong government said: “This is absolutely unacceptable to Hong Kong as a society that observes the rule of law,” referring to the acts of the attackers as well as the protesters.
China on Monday harshly criticised a demonstration in which eggs were thrown at its office in Hong Kong and messages spray-painted on the exterior walls.
The official People’s Daily newspaper said in a front page commentary that the protesters’ actions were “intolerable.”
The article headlined Central Authority Cannot be Challenged expanded on a strong condemnation issued the previous night by the government’s Hong Kong and Macao Affairs Office.
One group of protesters targeted China’s liaison office in Hong Kong on Sunday night after more than 100,000 people marched through the city to demand democracy and an investigation into the use of force by police to disperse crowds at earlier protests.
Hong Kong police launched tear gas at protesters at the pro-democracy march.
The march reached its police-designated end point in Hong Kong’s Wan Chai district in the late afternoon, but thousands continued, at various points occupying key government and business districts. They then headed for the Liaison Office, which represents China’s Communist Party-led central government within the city.
The country's national emblem, which hangs on the front of the building, was splattered with black ink. It was replaced by a new one within hours.
Organisers said 430,000 people participated in the march, while police said there were 138,000 during the procession’s “peak period”.
A group of pro-China lawmakers held a news conference Monday appealing for a halt to the violence, saying it was a blow to Hong Kong's reputation and was scaring away tourists and investors.
They also urged police to tighten enforcement against the protesters, whom Regina Ip, a former security secretary, called "rebels."
"The violent attack on the Liaison Office ... is a direct affront to the sovereignty of our country," Ip said.
Asked why it took at least a half-hour for police to arrive at the suburban train station where protesters were attacked, she said the police were "overstretched."
"The police have been under extreme pressure," she said.
Large protests began early last month in Hong Kong in opposition to a contentious extradition bill that would have allowed Hong Kong residents to stand trial in mainland China, where critics say their rights would be compromised.
Some are also calling for her to resign amid growing concerns about the steady erosion of civil rights in the city.