HONG KONG - Anger soared in Hong Kong on Monday over a vicious assault on pro-democracy protesters by suspected triad gangsters that left dozens wounded, a dramatic escalation of the political turmoil plaguing the Chinese city.

The financial hub's roiling unrest took a dark turn late Sunday when gangs of men -- most wearing white T-shirts and carrying bats, sticks and metal poles -- set upon anti-government demonstrators as they returned from another huge march earlier that day.

Footage from the attack broadcast live on Facebook showed people screaming as the men beat multiple protesters and journalists in Yuen Long station and inside subway trains, leaving pools of blood on the floor.

Hospital authorities said 45 people were wounded in the attack, with one man in critical condition and five others with serious injuries.

Critics rounded on the city's embattled police force, accusing officers of taking more than an hour to reach the station despite frantic calls from those under attack, and then failing to arrest the armed assailants who stayed in the streets around the station into Monday morning.

Some men in white shirts were later filmed leaving the scene in cars with Chinese mainland number plates.

Lam Cheuk-ting, a pro-democracy lawmaker, was one of those wounded in the melee, sustaining lacerations to his face and arms.

He criticized police for their response and accused "triad members" of being behind the attacks.

"Their very barbaric and violent acts have already completely violated the bottom line of Hong Kong's civilized society," he told reporters early Monday.

Furious fellow pro-democracy lawmakers held a press conference on Monday where they accused the city's pro-Beijing leaders of turning a blind eye to the attacks.

"This is triad gangs beating up Hong Kong people," fumed Alvin Yeung. "Yet you pretend nothing had happened?"

City leader Carrie Lam was due to hold a press conference later Monday alongside her police chief.

The clashes have ratcheted up concern that the city's feared triad gangs are wading into the political conflict.

Yuen Long lies in the New Territories near the Chinese border where the criminal gangs and staunchly pro-Beijing rural committees remain influential.

Similar assaults by pro-government vigilantes against demonstrators during the 2014 "Umbrella Movement" protests were blamed on triads.

Hong Kong has been plunged into its worst crisis in recent history by weeks of marches and sporadic violent confrontations between police and pockets of hardcore protesters.

The initial protests were lit by a now-suspended bill that would have allowed extraditions to mainland China, but they have since evolved into a wider movement calling for democratic reforms, universal suffrage and a halt to sliding freedoms in the semi-autonomous territory.

The city's parliament was trashed by protesters earlier this month, as Beijing's authority faces its most serious challenge since Hong Kong was handed back to China in 1997. -AFP