The storm has affected 3.5 million people on the Chinese mainland.
Trains from Guangzhou to Beijing have been suspended and hundreds of flights from Guangzhou, Shenzhen and Hong Kong have been cancelled.
However Hong Kong has escaped the worst of the storm.
Weather officials say that the ferocity of the storm has abated as it progressed into southern China, but financial markets in Hong Kong were closed for part of Monday morning.
More than 80,000 people were moved to safety in Fujian province and the authorities have deployed at least 50,000 relief workers, the state-run Xinhua news agency reported. Power supplies in many parts of the province and in Guangdong have been cut off.
"It is the strongest typhoon I have ever encountered," Xinhua quoted Luo Hailing, a petrol station attendant in Shanwei - in the eastern part of Guangdong province - as saying. "[It was] so terrible, lucky we made preparations."
Hong Kong`s port - one of the world`s busiest - shut down as the densely populated territory braced itself for the storm - the most powerful of this year.
Although officials say that Hong Kong escaped the worst ravages of the weather, the South China Morning Post said that Usagi still caused disruption and disarray, bringing flooding to some areas.
Just after 18:00 BST on Sunday the Post reported that Usagi`s designation had been reduced from Severe Typhoon to Typhoon and it was moving away from Hong Kong.
The storm killed two people on Friday as it crossed the Luzon Strait between Taiwan and the Philippines.
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