Those charged under the new law could be tried on the mainland. Legal experts say the measure ends any remaining autonomy the region enjoys under Chinese rule.
Despite fierce international criticism and opposition in Hong Kong, Beijing's rubber-stamp legislature passed a law allowing the mainland to impose security measures in the former British colony.
Boris Johnson wrote in a Times of London column that the law would infringe on the "one country, two systems" agreement China reached with Britain in 1997 when Britain ceded control of the territory.
The rally has been held each year since 1990 to commemorate the protest in Tiananmen Square, where the Chinese military opened fire on citizens who were calling for economic and democratic reforms.
Hong Kong is supposed to be a semiautonomous enclave of China, "one country, two systems." But China is moving to impose a sweeping national security law on the city, which critics say is illegal.
The National People's Congress authorized lawmakers to draft and enact a national security law aimed at suppressing dissent in Hong Kong. The U.S. says it means the end of the city's sovereignty.
Relations between the economic giants have gone downhill fast since they signed a preliminary trade pact four months ago. The latest tussles over the coronavirus and Hong Kong add to the friction.