A powerful typhoon has hit Hong Kong and the southern China coast, killing at least 20 people on the mainland, crippling power lines and causing flooding and gale force winds.

Typhoon Usagi, the strongest storm to hit the western Pacific this year, began pounding the Asian financial centre late last night.

More than 370 flights were cancelled.

The signal warning remained in force early this morning, with financial markets closed for at least part of the morning.

The weather observatory said the storm had weakened from "super" typhoon status and that it would consider lowering the warning signal.

China's National Meteorological Centre issued its highest alert, with more than 80,000 people moved to safety in Fujian province and authorities deploying at least 50,000 disaster-relief workers, state Xinhua news agency reported.

At least 20 people were killed on China's southern coast, television reports said, including 13 in Shanwei in the eastern fringes of Guangdong province.

The victims included people hit by debris and others who had drowned. One man was killed by a falling window pane.

Winds of more than 180km/h were recorded in some parts of southern China, toppling trees, cranes and blowing cars off roads in some areas.

The storm had earlier brought down three major power lines in coastal Fujian, cutting off electricity supplies to about 170,000 households, Xinhua reported.

In Guangdong province, a major base for Chinese nuclear power, the Daya Bay nuclear power plant just east of Hong Konghad initiated emergency response schemes.

Four of the six power-generating units at the plant had been ordered to operate at reduced load.

Airlines cancelled flights to cities in southern Guangdong and Fujian, while shipping was suspended between China and Taiwan.

Despite earlier warnings the typhoon could pose a severe risk to Hong Kong, the city suffered only minimal damage, including toppled trees. There were no fatalities in the city.

The Hong Kong Exchange delayed the start of trading on securities and derivatives markets due to the typhoon.

Usagi lashed the east and south coasts of Taiwan on Saturday after slamming into the Philippines' northern islands, where it cut communication and power lines and triggered landslides.