Thousands of demonstrators have been marching through the city centre. Police say 120 have been arrested.
Earlier, 200 senior politicians from around the world issued a joint statement criticising China’s plan.
Signatories called it a “comprehensive assault on the city’s autonomy, rule of law and fundamental freedoms”.
China is seeking to pass a law that would ban “treason, secession, sedition and subversion” in the territory.
It has dismissed fears the legislation would harm foreign investors in Hong Kong, an important financial centre, and has lashed out at “meddling” countries.
Hong Kong’s leader Carrie Lam, who is seen as part of the pro-Beijing political establishment, has pledged full support for the proposed law and said the city’s freedoms would remain unchanged.