The arrests come in the same week that lawmakers are due to vote on a controversial political reform plan.
Police spokesman Ng Wai-hon did not make a direct connection between the arrests and the vote but said at least one of the suspects belonged to a "radical group".
Hong Kong has seen large-scale protests for greater democracy in recent months.
In April Hong Kong's government unveiled a highly controversial proposal for selecting the city's next chief executive in 2017 which would involve candidates being vetted by a committee made up largely of pro-Beijing members.
Democracy activists have said they are opposed to the proposals and pro-democracy legislators have vowed to vote them down.
Thousands of people barricaded parts of Hong Kong for more than two months in protest when the proposals were first announced last year.
Police said items seized in the raids included air rifles, a formula for making smoke grenades, face masks and maps of locations in central Hong Kong.
Police said some of the substances were seized at an abandoned TV studio with other raids taking place at suspects' houses.
Pro-democracy activists, some of whom are known as "localists" because of their anti-Beijing stance, have distanced themselves from the alleged plot.
"Police said localist activists are making bombs, but I am not sure if it's real or not. We have nothing to do with that," Jon Ho of the Hong Kong Localism Power group told AFP.
Police in Hong Kong say they have arrested nine people on suspicion of conspiracy to manufacture explosives.