At least 50 people including rugby players were involved in a vicious street brawl on the eve of the Hong Kong Sevens, a report said on Saturday, casting a shadow over the city's biggest annual party.

Police used riot shields, batons and pepper spray to subdue running battles through the sleazy Wanchai area in the early hours of Friday, the South China Morning Post said.

Former Fiji Sevens player Paula Maisiri, who had been taking part in a 10-a-side tournament, was reportedly fined $HK500 ($A90) for assaulting a police officer.

"The fighting exploded like it would on a rugby pitch. There were so many people watching. I've never seen anything like it," an eyewitness was quoted as saying.

"There were police with shields and batons and blood everywhere," another said.

Despite widespread heavy drinking and rowdy behaviour, serious violence is unusual at the Hong Kong Sevens which attracts thousands of fans from all over the world and a large number of expatriate residents.

In 2005, a British barman had fingers and part of his right hand cut off in an attack outside the Sevens by Triad gangsters wielding meat cleavers and wearing surgical masks, who mistook him for somebody else.

The latest incident will be unwelcome for many bar and restaurant owners who have already been hit by the city's recession and are looking forward to one of their busiest weekends.

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