Police said on Sunday that they've seized a semi-automatic Glock pistol and 105 bullets in an operation on Hong Kong Island, arresting eleven people in the process.
Police said eight those arrested are men, and they're aged between 20 and 63.
Organised crime and triad senior superintendent Li Kwai-wah said it's the first time that a handgun has been seized in half a year of protests, and officers are very concerned.
He said they also seized two bulletproof vests after searching three premises -- on Heard Street in Wan Chai, Fortress Hill Road in Fortress Hill and Chai Wan Kok Street in Tsuen Wan.
Li said officers believe the suspects wanted to use the guns on police, to cause havoc and then "claim that it was the police who did it" at the anti-government march on Sunday afternoon.
The detective said three out of five of the magazines were already loaded with bullets, and forensic tests showed that the gun was in good condition.
Li said some of the arrestees were involved in a "radical group" that allegedly took part in a petrol bomb attack on the Mong Kok police station in October.
He also said some daggers, sabres, batons and pepper sprays were seized in the search.
The detective said officers only found the handgun this morning and they're investigating how the suspects got hold of it.
Li said participants in the march on Sunday should be careful, and inform marshals or the police if they see any potential threat.