Schools and businesses were shut as activity in the normally teeming financial hub slowed to a crawl after Usagi punched a long swathe of Chinese coast with torrential rain and winds of up to 165 kilometres (103 miles) per hour during the night.
The deaths were reported by Chinese state media after Usagi -- which meteorologists say was the world's most powerful storm this year -- made landfall in Guangdong province northeast of Hong Kong yesterday evening, prompting the highest level of alert from the National Meteorological Center.
Bullet trains from Guangzhou city to Beijing were suspended and Xinhua said that winds were strong enough near Shanwei to blow cars off the road. More than 47,000 fishing boats were in harbour and schools were closed in 14 coastal cities.
Usagi had previously killed two people in the Philippines and unleashed landslides and power outages across southern Taiwan at the weekend as it ploughed through the Luzon Strait with ferocious winds and heavy downpours.
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As the typhoon bore down on Hong Kong, operators shut down one of the world's busiest sea ports and nearly 450 flights were either cancelled or delayed as Cathay Pacific and other airlines imposed preemptive suspensions.
The Observatory said it was the strongest typhoon to brush Hong Kong since 1979. Tens of thousands of people had their travel plans upended with ferries and trains also disrupted, while Cathay said it expected flights to start resuming only from noon (0400 GMT) on today.
Many passengers were milling around Hong Kong airport hoping to rebook their flights, but hand-written signs in Chinese and English warned them that there was little chance of getting standby seats on flights out today and to check back later.
Major thoroughfares were empty and signboards swayed in the wind early today, but some residents ignored official warnings and headed out to the coastline in raincoats to brave the wind.