More than 8,000 police officers have been deployed across the city according to public broadcaster RTHK, citing people familiar with the matter.
A small group of pro-democracy activists took to the streets, but were surrounded by officers while they marched and chanted.
Changes to the political system are the latest in a string of moves - including a national security law that punishes anything Beijing deems as subversion, secession, terrorism, or collusion with foreign forces - that have placed the international financial hub on an authoritarian path.
China's rubber-stamp parliament in May changed Hong Kong's electoral system, reducing democratic representation in institutions and introducing the vetting mechanism for election candidates and winners. This all but removed any influence the opposition was able to exert.