Hong Kong police arrested eight people, including a prominent political activist, suspected of possessing offensive weapons, fuelling further anger among the millions of anti-government protesters who have taken to city streets all summer.

Police raided an industrial building on Thursday and arrested seven men and a woman, including Andy Chan, the founder of the Hong Kong National Party, a political group banned by city authorities last September.

Supporters gathered late at night at a police station in the neighbourhood of Sha Tin, where the suspects were thought to be held, egging the building and chanting, “Free the martyrs!”

Officers also raided a flat and seized 30 smoke bombs, though it remains unclear what the explosives were for and whether the cases were related. Earlier this week, someone shot fireworks at protesters gathering outside another police station in a drive-by attack.

Hong Kong is embroiled in its worst political crisis since the former British colony was returned to Chinese rule in 1997.