Incredible footage of Typhoon Hato has shown the maximum category 10 storm slamming into Hong Kong with force that has overturned trucks and blown people down streets like rag dolls.
Wind and rain is lashing the Asian financial hub, uprooting trees and forcing most businesses to close - while in Macau three people were killed.
There were reports of 34 people injured in Hong Kong while more than 450 flights were cancelled, financial markets suspended and schools closed as Hato bore down, the first category 10 storm to hit the city since 2012.
"I've never seen one like this," Garrett Quigley, a longtime resident of Lantau island to the west of the city, said of the storm.
"Cars are half submerged and roads are impassable with flooding and huge trees down. It's crazy."
Many skyscrapers in the usually teeming streets of Hong Kong were empty and dark as office workers stayed at home.
In residential districts such as Heng Fa Chuen on densely populated Hong Kong island, waves smashed against the sides of oceanfront buildings and surged over a promenade, sweeping away walls and benches and swamping vehicles parked nearby.
Construction cranes swayed at the tops of skyscrapers, windows imploded and nearly 200 trees were uprooted, while some people used canoes to venture out into flooded streets.
Authorities downgraded the storm to a category three by late-afternoon with government services, the courts, financial markets and companies set to resume normal business on Thursday.
The storm also caused a power blackout across most of the gambling hub of Macau for about two hours, residents said, with disruption to mobile phone and internet networks. There was severe flooding on the streets, with some cars almost completely submerged, and the water supply was affected in some districts.
The three men who died included a 45-year-old Chinese tourist who was hit by a heavy truck, according to a government statement.
The former Portuguese colony's casinos, however, had backup power, two casino executives told Reuters.
The storm also made landfall in China's Guangdong province, in Zhuhai city adjacent to Macau, Chinese state news agency Xinhua reported.
Numerous flights and trains were cancelled in Guangdong province, with Shenzhen's International Airport particularly badly hit.
Thousands of residents along the Chinese coast were evacuated and fishing vessels were called back to port.
Maximum winds near Hato's centre were recorded at a destructive 155km/h as it continued to move west across Guangdong in the general direction of Hainan island.
A senior scientific officer for the Hong Kong observatory warned that sea levels could rise several metres in some places, with the government issuing flood alerts and opening 27 shelters across the city.