Hong Kong police said on Sunday they seized a semi-automatic pistol, ammunition and other weapons and have arrested 11 people during overnight raids ahead of a major protest in the city.

Police said they seized a Glock semi-automatic pistol and four magazines, three of which were loaded with a total of 105 bullets, bottles of pepper spray and knives.

It is the first time a handgun has been seized during the ongoing protests, according to Senior Superintendent Li Kwai-wah of Organized Crime and Triad Bureau.

The police received information suggesting that some members of a radical group, who had participated in an unlawful assembly on October 20 and attacked the Mong Kok police station with a large number of petrol bombs, were planning to use weapons including guns to create chaos and attack the police during Sunday's demonstration, Li said at a media briefing.

The police therefore launched an operation in the morning at 11 locations in various districts around Hong Kong.

A total of 11 people, including eight males and three females, have been arrested in the operation for offenses including possession of a firearm without license, possession of dangerous goods, possession of prohibited weapons and unlawful assembly.

Li said the police are investigating where the weapons came from and may make more arrests as the operation is still ongoing.

He has also warned organizers and participants in Sunday's demonstration to watch out for their personal and public safety.