Police said one suspect had claimed to be a member of a "radical local group" but would not name the group or specify motives.
The bomb allegations have been met with scepticism by some commentators in the semi-autonomous Chinese city, who questioned their timing before the vote expected by Friday.
A senior Chinese official warned democrats to tread a "moderate" path.
"The radical opposition camp has recently resorted to despicable means... And are engaged in extreme violent activities," said Song Ruan, deputy commissioner for Beijing's foreign affairs office in Hong Kong, without specifically referring to the arrests.
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Song urged lawmakers to pass the bill setting out how Hong Kong will choose its next leader in 2017.
Democrats in the city's legislature have said they will vote against the plan, denying it the two-thirds majority it needs to pass.
Although it will for the first time give all residents the right to vote for the chief executive, it adheres to a Beijing ruling that candidates must be vetted by a loyalist committee.
That ruling sparked mass protests and street blockades late last year by people decrying "fake democracy".
China has not made clear what its response will be should the package be vetoed.
"No-one knows when the constitutional reform process will be re-launched," said Song.
"The prospects for Hong Kong will be very worrying."
Daily rallies are being held before this week's vote. Police warned anyone taking part to stay away from "violent protesters" following yesterday's arrests.
All 10 suspects were arrested "on suspicion of conspiracy to manufacture explosives" and detained overnight.
Police also seized chemicals at an abandoned television studio in the eastern district of Sai Kung, with some detonated at the scene.