Hong Kong's top typhoon warning will be downgraded at 1.20pm, the observatory said on Wednesday, after being in force for 10 hours, 40 minutes – the second-longest on record.

Only Typhoon York, which struck the city in 1999, resulted in a T10 that lasted longer – by just 20 minutes.

Super Typhoon Ragasa created havoc across the territory, felling hundreds of trees and sending dozens of people to hospital.

"With Ragasa moving towards the coast of western Guangdong, its associated hurricane force winds will also depart from Hong Kong gradually," the observatory said.

"The Observatory will issue the No. 8 Southeast Gale or Storm Signal at 1.20pm today."

Forecasters nonetheless warned that the weather will be persistently adverse on Wednesday with frequent heavy squally showers and thunderstorms, while seas will be "phenomenal with swells."

"There will be overtopping waves over the shoreline, which will be particularly significant along the eastern and southern coasts."

The storm surge caused water levels to rise by 3.4 metres or more above Chart Datum on Wednesday morning, and the observatory says waters on the west coast will still rise slightly for the next several hours.

Authorities received 350 reports of fallen trees, 12 reports of flooding and one landslide as of 11am, while the Hospital Authority said 56 people had sought treatment at accident and emergency units at public hospitals.