Runners and organisers of the Hong Kong Oxfam Trailwalker on Friday braced for a rainy start and a mischievous troop of monkeys.

An online video showed that dozens of monkeys were raiding a supply station set up along the route, ransacking for food and delving into a torn blue-and-white striped nylon bag.

Michael Wong, Oxfam Hong Kong’s director of fundraising and communications, said it is hard to prevent the animals from enjoying what he called a "once-a-year, all-you-can-eat buffet" at the checkpoint.

Wong said volunteers could only replenish food and supplies at the station.

The Oxfam Trailwalker is one of the city's largest fundraising events.

Participants are given 48 hours to complete the 100-kilometre run, kicking off at Pak Tam Chung in Sai Kung and finishing at Harrow International School Hong Kong in Tuen Mun.

There is also a shorter 40-kilometre race.