HONG KONG - Hong Kong's largest labor group on Sunday strongly condemned the violent protests that paralyzed traffic on Hong Kong Island, forcing it to cancel a scheduled gala to celebrate National Day.

The Hong Kong Federation of Trade Unions had to scrap the cultural event on public safety concerns after protesters again resorted to violence and vandalism following an unauthorized procession on Sunday afternoon.

Federation Chairman Wong Kwok said rioters, who have instigated violent confrontations across the city ahead of National Day, have breached the bottom line of "one country, two systems" in a blatant challenge to national sovereignty.

He said it's a crying shame that protesters have used reckless vandalism and sabotage, including burning National Day decorations, before celebrations marking the 70th anniversary of the founding of the People's Republic of China.

The federation announced the cancellation of the gala hours before it was due to be held at Queen Elizabeth Stadium in Wan Chai at 7pm, saying the safety of both performers and the public would be at risk.

Chief Executive Carrie Lam Cheng Yuet-ngor last Thursday held her first community dialogue session at the same venue, which was besieged by protesters.

A fresh round of violence wreaked havoc on Hong Kong Island on Sunday, stretching from the Central business district to Causeway Bay.

Thousands of radical protesters defied a police objection to the procession, obstructing thoroughfares, hurling bricks and petrol bombs at the police, setting fires to makeshift barricades on the streets and vandalizing metro stations.

"These reckless acts have destroyed the city's peace and order, as well as economic activities," the federation said in a statement, urging Hong Kong residents to support the police in law enforcement.

Wong said the turmoil has deprived the public of their right to cultural entertainment, adding that the cancellation is unfair to performers who had rehearsed for a long time, and will affect cultural exchanges between Hong Kong and labor unions on the mainland.