Recent court judgments have freed Hong Kong authorities to use national security powers to deploy tough colonial-era laws in a crackdown 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.