WATCH: Aftermath of typhoon Saola in Hong Kong

Typhoon Saola swept across southern China on Saturday after tearing down trees and smashing windows in Hong Kong, although the megacity avoided a feared direct hit from one of the region's strongest storms in decades.

Tens of millions of people in the densely populated coastal areas of southern China had sheltered indoors on Friday ahead of the storm.

Saola had triggered the city's highest threat level in Hong Kong on Friday evening - which had only been issued 16 times since World War II.

It was downgraded before dawn on Saturday after the typhoon passed the city, with no reported casualties, and tracked towards coastal areas of mainland China.

However, authorities warned people to remain on alert, with strong winds and storm surges still a threat.

"Gale force winds are still affecting some places... precautions should not yet be relaxed," said the city's weather observatory at 8am.

Saola was still packing sustained winds of 145 kilometres per hour at its centre, creating a storm surge that caused rough swells around Hong Kong's waterfront areas.

The last time Hong Kong issued a T10 warning was in 2018, when Typhoon Mangkhut slammed into the city, shredding trees and unleashing floods, leaving more than 300 people injured.

In mainland China, Mangkhut killed six people and impacted the lives of more than three million others.