Hong Kong police arrested nine people and seized suspected explosives, authorities said Monday as the city went on high alert ahead of a crucial vote on a China-backed electoral reform package that sparked widespread protests last year.

The overnight raids involving scores of officers came three days after police warned they were monitoring online activity to track anyone planning to incite violence. Dozens were on patrol Monday near government headquarters, where metal fences have been erected, before debate on the package begins Wednesday.

The nine suspects were arrested on suspicion of conspiracy to manufacture explosives, police said.