At least two political rights groups advocating democracy have quietly quit Hong Kong and moved overseas, unnerved by a national security law that has fanned fears over the erosion of freedoms under China’s rule, sources told Reuters.

In the past, China-focused rights groups had valued the wide-ranging autonomy, including freedom of speech and assembly, guaranteed for Hong Kong when control over the former British colony was returned to Beijing in 1997.

But some non-government organisations (NGOs) say the new legislation means they face...