A vicious attack on pro-democracy protesters by suspected triad gangsters on Sunday night in a Hong Kong metro station was premeditated and likely planned days in advance, a legislator assaulted in the incident has claimed.

Lam Cheuk-ting, a Democratic Party politician, was cornered by stick-wielding thugs who fractured his hand and tore a deep gash in his face requiring 18 stitches as he tried to defend terrified commuters from the rampaging gang in Yuen Long metro station in Hong Kong’s New Territories.

His claims about the lawless mayhem at the station will compound pressure on Hong Kong’s embattled police force, which is already under fire from a public deeply shocked by the violence and angered by the authorities’ slow response as it unfolded.

In a Telegraph interview, Mr Lam alleged that anonymous internet warnings appeared “two or three days before the attack” that said it would be “dangerous if anyone has a black shirt and if they go to Yuen Long.” Black is the colour widely worn at Hong Kong’s recent mass protests.

Postings claimed residents of a nearby village were told to avoid the area on Sunday, he said. While early that night, people leaving the station spotted hundreds of gangsters wearing white shirts and tried to call the police. “The police urgent hotline was disconnected,” reported Mr Lam.