Hong Kong is in shock after a night of violence on Sunday. Dozens of masked men stormed a train station and attacked commuters.

The men - dressed in white shirts and suspected to be triad gangsters - assaulted pro-democracy protesters and passersby in the district of Yuen Long.

This is the first time this kind of violence has been seen in the ongoing anti-extradition demonstrations.

Several lawmakers pointed the finger at police accused to have reacted very slowly to the call by attacked commuters.

Footage posted on social media showed dozens of men attacking people with batons inside the station.

Forty-five people were injured, with one person in critical condition.

Lawmaker Lam Cheuk-ting said police had taken more than an hour to arrive.

"Hong Kong has one of the world's highest cop to population ratio," said another pro-democracy lawmaker Ray Chan in a tweet. "Where were they?"

Police on Monday said they had not made any arrests but were still carrying out investigations.

On Sunday afternoon there had been another rally.

Riot police fired tear gas and rubber bullets at protesters in Hong Kong at a large pro-democracy rally, and charged demonstrators who threw objects at police lines.

The protest route was altered with protesters told to stop at Wan Chai rather than Central, where the key government offices are located.

Some 4,000 police officers were deployed.

Organisers of Sunday's protest say more than 430,000 people took part but police put the figure at 138,000.