About a hundred people who gathered in a mall on Monday to watch the men's final in foil on a big screen booed when China's national anthem was played after the fencer Cheung Ka Long's gold.
And as part of the security laws introduced last summer in the semi-autonomous city, it is a criminal act to show disrespect for China's national anthem.
The action can result in up to three years in prison and the police state that they have initiated a preliminary investigation into the crime.
The security law itself has been criticized around the world as draconian, a way for the Beijing regime to dilute the freedoms enjoyed by Hong Kong people, rights that do not exist in the rest of China.