HONG KONG—Recent court judgements have freed Hong Kong authorities to use national security powers to deploy tough colonial-era laws in a clampdown against opposition groups, alarming activists and lawyers in the city. Police have launched investigations into acts that took place before the national security law was imposed a year ago, despite assurances by Beijing and Hong Kong that the financial hub’s legislation would not be retroactive. The recent probes have unnerved pro-democracy campaigners across the city, leaving some to fear they face prosecution for acts they believed to be legal at the time. “The past is the future,” said Simon Young, a professor at the University of Hong Kong’s law school. “We are starting to see a fuller evolution of the national security law, and the way it allows the authorities to look at older laws and past events through a new lens. “We can see it gives them …