* Petrol bombs thrown inside metro station - government
* Protesters take to the streets after week of relative calm
* Metro station seriously damaged but no injuries
* Riot police on streets of Kowloon (Adds latest on protests, fresh quotes)
By Noah Sin
HONG KONG, Oct 12 (Reuters) - Petrol bombs were thrown inside a Hong Kong metro station on Saturday but no one was injured, the government said, 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. Riot police took to the streets of Kowloon after the petrol bomb attack.
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.
"The protesters are participating in an unauthorized assembly, which constitutes an offence under the Hong Kong Laws. The police appeal to the protesters to stop all illegal acts and leave immediately," police said in a statement.
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.
'DEFEND THE FUTURE'
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".
"The young people have already sacrificed a lot, it is about time for us, the senior citizens in Hong Kong to come forward to take up part of the responsibility from the young people," 63-year-old Shiu told local media.
"I mean for us, even if we are caught by the police because of an illegal gathering, I don’t mind," said Shiu, who was identified with only one name.
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.