HONG KONG -- A viewing party for an Olympic event that produced Hong Kong's first gold medal in a quarter century has led to the arrest of a man accused of disrespecting China's national anthem.

The territory's police said Friday evening a 40-year-old man had been arrested on suspicion of violating the national anthem law, based on a video from Monday night, when he was watching local fencer Edgar Cheung Ka-long win the men's foil competition in Tokyo.

The police said the man, surnamed Leung, is a reporter, but did not give further details about his identity, including for what media organization he works.

This case marks the first arrest under the law, which punishes disrespect for the Chinese national anthem and the national flag. The maximum penalty is three years in prison, a 50,000 Hong Kong dollar ($6,400) fine, or both.

Eileen Chung Lai-yee, superintendent of the police in charge of organized crime and triads, told a news conference that the suspect raised the flag from the period of British rule over Hong Kong during the anthem. He is also accused of "inciting others on site to boo and chant slogans," according to the police.

The law went into force in June 2020, a few weeks before Beijing imposed its national security law on semi-autonomous Hong Kong following massive street protests that were sparked the year before by proposed changes to extradition rules. Friday's announcement came on the same day that the first person convicted under the national security law was sentenced to nine years in prison.

The man in the anthem arrest was part of a crowd, estimated at thousands of people, who flocked to a public viewing site at a shopping mall in Kwun Tong in Kowloon district to watch the fencing medal event. It was an emotional moment for Hong Kong, which brought an unusual sense of unity among people divided over political issues in recent years.

Cheung is the first Olympic gold medalist since the territory's handover from the U.K. to China in 1997, and only the second one after Lee Lai-shan won hers in sailing at the 1996 Atlanta Games.

Edgar Cheung Ka-long of Hong Kong, left, celebrates defeating Daniele Garozzo of Italy in the men's individual final Foil competition at the 2020 Summer Olympics on July 26 in Chiba, Japan.    © AP

Hong Kong's strict social distancing rules were not enforced that night by the police, which allowed people to gather for a single purpose for the first time in a long time. This is in sharp contrast to the rigorous policing used against pro-democracy events, such as the annual candlelight vigil commemorating the deadly 1989 Tiananmen Square crackdown. Authorities banned this gathering for a second straight year this year, citing the pandemic.

The territory's stark political differences resurfaced when Cheung stood on the podium to receive his gold medal. Viral video showed people at the viewing site booing while the Chinese anthem played, with some shouting, "We are Hong Kong!" An Instagram page showing the video had been viewed 225,000 times as of Friday evening.

A different video shared online replaced the Chinese national anthem with "Glory to Hong Kong," a protest song from the 2019 anti-government movement. The government considers part of the lyrics to be "separatist and subversive," making them illegal under the national security law.

Reporters asked Chung how the police had tracked down the suspect out of so many people, but the superintendent gave few clues.

"The problem is not only about raising whatever flag, but the overall conduct," Chung said.

"What he did and the reactions at that location was the way in which the national anthem was insulted, that is a conduct of not being respectful to the national anthem," she said.

Asked whether there would be further arrests on this case, another police officer at the news conference declined to comment on any future actions "since the investigation is ongoing."

Security Secretary Chris Tang Ping-keung, who oversees the police, vowed to investigate "any violations of the law, including the national [anthem] law or the national security law."

"If there is any evidence, we will arrest," Tang added. "And if there is further evidence, we will prosecute."

Additional reporting by Stella Wong and Cora Zhu.