Police said that the bomb measured approximately 1.5 metres (4.9 feet) in length and weighed about 450 kilograms (1,000 pounds), adding that they believed it remained "fully functional".

"Due to the extremely high risks involved in dismantling and disposing of the bomb, we must activate the emergency evacuation plan," said Hong Kong Police's district commander Andy Chan.

About 6,000 people from 18 residential buildings in the Quarry Bay area will be evacuated Friday night, with demolition work set to begin early Saturday morning, police added.

Hong Kong was an early target in what would become a full-blown Asian campaign for imperial Japan during the Second World War.

The city saw fierce fighting between Japanese and Allied forces during wartime, and hikers and construction workers still occasionally discover unexploded bombs in the territory nearly 100 years later.

In May 2018, a bomb was discovered in Hong Kong's Wan Chai district, which led authorities to evacuate 1,200 residents.

Police said the unexploded ordnance unearthed on Friday was of the same type as the one found in 2018.

At that time, the bomb disposal process took about 20 hours to complete.