Safeguard Hong Kong held up a giant Chinese flag and sang the national anthem.
It said it had formed a team of volunteers to make sure nobody desecrated the many national flags being flown across the city on National Day.
The group’s convenor, Kennedy Wong, urged people to treasure the “One Country, Two systems” principle and not to take part in illegal acts.
A number of national flags have been damaged during the recent protests, with some being ripped down, set on fire or thrown in the sea.
Last week, Safeguard Hong Kong called on the government to seek court injunctions to ban people from getting close to the flag poles of some 200 government buildings.