Petrol bombs were thrown inside a Hong Kong metro station on Saturday but no one was injured as pro-democracy protesters again took to the streets angry at what they believe is Beijing's tightening grip on the city.

The Kowloon Tong station was seriously damaged in the attack, the government said in a statement.

Hundreds of protesters, many young and wearing face masks, were marching in Kowloon at the time and were headed to a district near the Kowloon Tong station.

READ MORE Disappointment after Hong Kong protests force cancellation of Asia Pacific Deaf Games

About a dozen riot police took to the streets in Kowloon's Tsim Sha Tsui district, normally a haven for local and international shoppers, behind the marchers shortly after news of the petrol bomb attack.

Hong Kong's metro has borne the brunt of protests, with stations torched and trashed, and only returned to normal operations on Friday after being completely shut down.

The metro normally carries around 5 million people a day.

AP

Hong Kong's protests started in opposition to a now-abandoned extradition bill but have mushroomed in four months into a pro-democracy movement and an outlet for anger at social inequality in the Asian financial hub.

The protests have plunged the city into its worst crisis since Britain handed it back to China in 1997 and is the biggest popular challenge to Chinese President Xi Jinping since he came to power in 2012.

Hong Kong had experienced relative calm since last weekend when a peaceful march by tens of thousands spiralled into a night of running battles between protesters and police.

Since then there had only been small nightly protests and activists had not flagged any major action this weekend.

A small group calling itself the Silver-Haired Marchers began a 48-hour sit-in at police headquarters on Saturday, describing themselves as "old but not obsolete".

"Whilst we may not be able to fight alongside the young protesters in the frontline against an unjust government, escalating police violence and indiscriminate arrests, we take it to heart to uphold the core values of Hong Kong and defend the future of our younger generations," it said in a statement.

Colonial-era emergency laws were introduced a week ago banning face masks at public rallies, sparking some of the worst violence since the protests started. Protesters use masks to shield their identities.

However, hundreds of people, including school children and office workers, have since defied the ban and wore face masks.

A group of protesters plan a "face mask party" on Saturday night.