The measures will affect tens of millions of people and crimp activity at thousands of factories across a swathe of China's manufacturing heartland.

Ragasa is expected to make landfall in the central and western coastal areas of Guangdong within 24 hours, the province's emergency management bureau said in a statement on Tuesday morning.

"Key areas should decisively adopt measures... fully ensuring the safety of people's lives and property, and minimising disaster losses to the greatest extent," the statement said.

Guangdong triggered its highest emergency response for windstorms as Ragasa approached, packing gusts of up to 230 kilometres (140 miles) per hour, according to the weather service in nearby Hong Kong.

The technology hub of Shenzhen has ordered the evacuation of 400,000 people, with the city's emergency management authorities warning of "severe wind, rain, waves and floods".

"Except for emergency rescue personnel and those ensuring people's livelihood, please do not go out casually," the authorities said in a statement, adding that work and market closures would begin in the afternoon.

Other major cities implementing suspension measures included Zhuhai, Dongguan and Foshan.

"The strong winds and heavy rainfall... will severely impact our city, posing a critical defence situation," Foshan's emergency headquarters said in a statement.

Ragasa is currently churning across the South China Sea, having earlier hit parts of the Philippines, where one person was killed in the storm.

Vessels operating in the northern part of the waterway, as well as in the southern part of the Taiwan Strait, the Bashi Channel, and nearshore waters of Guangdong "should pay attention to safety", Chinese state broadcaster CCTV said.

Scientists generally agree that human-driven climate change is making storms more powerful and frequent as the world warms.