Hundreds gathered at a shopping mall on Monday to watch a broadcast and cheer on Hong Kong fencer Edgar Cheung, who won gold in the men’s individual foil.
When the Chinese national anthem was played, some in the crowd began jeering while others shouted “We are Hong Kong”, video filmed at the scene shows.
It is illegal to insult the anthem under a recently passed law.
Reports also said that the British colonial flag was flown and some had chanted protest slogans, which could possibly violate the national security law which forbids anything that incites “secession” and could result in life in jail.
Both laws were passed last year and have been met with huge controversy, with critics saying they clamp down on free speech.
But Hong Kong authorities and the Chinese government deny this and say the laws are necessary to preserve peace and patriotism.
Hong Kong saw widespread protests in 2019 when tens of thousands took to the streets demanding democratic reforms. Some of those demonstrations turned violent as protesters and police clashed.
Since then it has been ruled under the “one country, two systems” principle that preserves freedoms in the city that the mainland does not have.