HONG KONG—Hong Kong’s national security police on July 12 arrested five more people in relation to an alleged bomb plot, as political tensions rise amid Beijing’s tightening control over the city. The five—four males and one female—were arrested on suspicion of conspiring to plan terrorist activities under a sweeping national security law imposed by Beijing a year ago as part of a clampdown on dissent in the former British colony, which had enjoyed freedoms not experienced on the Chinese mainland. Last week, police arrested nine people, including six students, accused of planning to manufacture and plant explosives around the city, including in courts, cross-harbor tunnels, and public trash cans. Police said then that they found chemicals in a makeshift laboratory for the manufacture of the explosive triacetone triperoxide, or TATP, which has been widely used in bombings in Europe and elsewhere. Police said they did not rule out further arrests …