Siobhan Haughey became Hong Kong's first ever multiple Olympic medal winner on Friday as she won her second silver of Tokyo 2020 in the women's 100m freestyle.

Haughey was already Hong Kong's first ever Olympic medallist in swimming after she clinched a silver medal in the 200m free on Wednesday.

Her double success has helped Hong Kong take their medal tally in the entire history of the Olympics from three to six, after Edgar Cheung Ka-long also won a fencing gold on Monday.

Hong Kong's previous three medals came in sailing, table tennis and cycling.

Born in Hong Kong to an Irish father and Hong Kong mother, Haughey grew up in the region before moving to America to attend university in Michigan. She is the grand-niece of former Irish prime minister Charles Haughey.

The 23-year-old, who paints her nails before competitions, was pipped to the gold in the 100m by Australia's Emma McKeon, whose time of 51.96 seconds was an Olympic record and the second fastest in history.

Haughey's 52.27 smashed her own personal best while setting a new Asian record. "This is crazy and surreal," she said. "The 200m freestyle is always my main event, so this is just a bonus.

"My goal is just to go in and have fun and swim a best time and I did that, and I also got another silver medal."

Haughey could yet win a third medal in the 50m freestyle, which starts later on Friday. 

In terms of number of medals, she is already Hong Kong's most successful ever Olympian.

"I don't know about that, but it is crazy to think that, I don't even know what to say," she said.

"I'm just here having a good time, and if that also means having great results it is so much fun."

Haughey was beaten by another Australian, Ariarne Titmus, in the 200m freestyle on Wednesday, despite leading the race until the final 25m.

But she was always playing catch-up in the 100m final as McKeon was first at the turn and powered through to leave Haughey and Australia's Cate Campbell fighting for second.

Haughey's achievements have captured the imagination of fans in Hong Kong, with hundreds of people gathering in a shopping mall to watch her race on a big screen.

Earlier on Friday, Hong Kong police said they had launched an investigation into fans booing China's national anthem during Cheung's medal ceremony on Monday.

Hong Kong chief executive Carrie Lam said in a statement Haughey had made her compatriots proud.

"Haughey has fully demonstrated great skill and perseverance to stay ahead of competitors, achieving an excellent result for Hong Kong," Lam said. 

"She has brought glory to the city and has made me and Hong Kong people proud."