HONG KONG—A powerful typhoon swept through China's southern Guangdong province on Sunday, killing at least 25 people and prompting the cancellation of hundreds of flights, before being downgraded to a tropical storm on Monday morning, officials said.

Typhoon Usagi doused the province's Pearl River Delta region with heavy rains and high winds that blew cars off the road, led to the evacuation of thousands of people and widespread electricity outages.

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