The action follows a new China national security law for Hong Kong that bans treason and other perceived offenses in the semi-autonomous territory.
The US secretary of state says the territory no longer merits special treatment under US laws.
The clashes come as parliament debates a bill to criminalise insulting the Chinese national anthem.
U.S. President Donald Trump said on Tuesday he is preparing to take action against China this week over its effort to impose national security laws on Hong Kong, but did not give details. "It's something you're going to be hearing about ... before the end of the week - very powerfully I think," Trump said in response to a second question.
Beijing says it will improve its preparedness for military combat. In the background: heightened U.S. tensions.