About 6,000 people were being evacuated from Quarry Bay on Friday evening as police prepare to carry out a bomb disposal operation overnight, following the discovery of a Second World War explosive at a construction site.

Officers said the wartime device found on Pan Hoi Street is an unexploded American aerial bomb, weighing some 1,000 pounds.

It contained around 500 pounds of explosives and is of a similar model unearthed in 2018 at a construction site for the Sha Tin to Central Link.

"The bomb is believed to be functional and extremely dangerous... We have decided to dispose of the bomb on site. Due to the extremely high risk in removing the device, we have to activate an emergency evacuation plan," said Eastern district commander Andy Chan.

People from 18 nearby residential and commercial buildings will have to evacuate by 11pm, before the removal operation begins at 2am.

Officers are hoping to get rid of the ordnance within 12 hours.

Suryanto Chin-chiu, a senior bomb disposal officer, said one way to handle the device is to cut through its casing and then burn off the explosives inside.

"If we do not properly handle [the bomb] or if we underestimate the risk, its destructive power – as designed during the Second World War – could cause massive casualties and damage to people and buildings," said Suryanto.

"We will not detonate [the bomb], because the nearest residential buildings are within a very short distance. If we detonate it on the spot, the nearby residential buildings will certainly be severely impacted."