North Korea won the Asian Games women's soccer gold medal for a third time on Wednesday, beating 2011 World Cup winners Japan 3-1 at Munhak Stadium.
The Koreans took a 2-0 lead on goals from Kim Yun Mi and skipper Ra Un Sim before Japan hit back through captain Aya Miyama. Substitute Ho Un Byol scored a brave diving header in the closing minutes to seal the win.
It was the third straight Asian Games final played between the two sides after Japan won four years ago in Guangzhou and the Koreans won in Doha in 2006.
Despite the ongoing political tension in Northeast Asia, the game was played in a festival-like atmosphere with South Korean spectators wearing T-shirts calling for harmony on the Korean peninsula and chanting for unification.
Late in the second half, however, plain-clothed security personnel swooped on a man who unfurled a North Korean flag and hung it on barriers directly behind the Japanese goal.
Security yanked the flag down and whisked the man away for questioning.
A Reuters witness heard the man, who looked to be in his 50s, tell security in Korean: "I didn't know this was against the law".
When asked his nationality, he handed over a burgundy-colored passport. Standard South Korean passports are green.
The man was photographed before being led away.
The country's strict National Security Act prohibits South Koreans from carrying the North's flag and the government issued a stern warning ahead of the Games that it was banned.
Asiad organizers also took down North Korean flags in Incheon ahead of the Games following complaints from conservative South Korean groups.
North Korea will have the chance to win gold in the men's event on Thursday when they play hosts South Korea.