"The Organised Crime and Triad Bureau has conducted an operation and discovered a certain amount of suspected explosives in (eastern) Sai Kung," a spokesman told AFP.
A police source told AFP that "there were arrests" related to the explosives, but would give no further detail.
Media reports said that nine people had been arrested in the operation in the east-coast district of Sai Kung, and that the explosives had been found at an abandoned television studio.
The Post said police suspect the explosives "were intended to be detonated before the Legislative Council debates the government's political reform package this week."
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The reform bill lays out a plan for choosing the city's next leader by public vote for the first time in 2017.
But it sticks to a ruling from Beijing which stipulates that candidates must be vetted by a loyalist committee.
That ruling sparked mass rallies towards the end of last year, with campaigners dismissing it as "fake democracy'.
Hong Kong is semi-autonomous after being handed back to China by Britain in 1997 and sees much greater freedoms than on the mainland, but there are fears that those are being eroded.
"Localist" groups are frustrated with the lack of progress on electoral reform and have argued that Hong Kong should distance itself from Beijing to forge its own political future.
Two localist groups told AFP that they had no knowledge of the arrests and that they did not condone violence.
People Power's Tam Tak-chi added: "People Power did not do that. Our group does not believe in violence.