The powerful storm posed a wide threat to lives and homes, prompting authorities in mainland China to order businesses and schools to shut down in at least 10 cities across the nation's south, affecting tens of millions of people.

Ragasa had already toppled trees, torn the roofs off buildings and killed at least two people in a landslide while lashing the northern Philippines, where thousands sought shelter in schools and evacuation centres.

The super typhoon was generating maximum sustained winds of 220 kilometres (137 miles) per hour near its centre while moving west across the South China Sea, according to Hong Kong's weather service.

The Hong Kong Observatory issued its third-highest typhoon warning, T8, at 2:20 pm (0620 GMT) on Tuesday, saying that Ragasa "has an extensive circulation with fierce winds".

"The weather is expected to start deteriorating rapidly later today, with winds strengthening quickly," the observatory said, adding that "significant" storm surge may cause water levels to rise up to four metres by Wednesday morning.

Yang Lee-o, who has lived in the seaside neighbourhood of Lei Yue Mun for 40 years, said government workers had already spent a day placing sandbags.

"Lei Yue Mun is the hardest hit whenever there's a typhoon or rainstorm," said the 71-year-old, adding that the water level went up to her thighs during a previous super typhoon.

Scores of workers have set up barricades and elevated walkways, with some business owners moving their wares out of the area.

Hong Kong's number-two official, Eric Chan, previously said Ragasa would pose a "serious threat" comparable to the super typhoons of 2017 and 2018, which cost hundreds of millions in property damage.

Scientists warn that storms are becoming more powerful as the world warms due to the effects of human-driven climate change.

Hong Kong's airport will remain open, but there will be "significant disruption to flight operations" from 6:00 pm (1000 GMT) Tuesday until the next day, the Airport Authority said.

More than 500 Cathay Pacific flights are expected to be cancelled.

Student Lilly Rober, 24, said she had plans to travel to Japan but her flight was pushed back.

"We had to book a new hotel here, which also costs us some money so we are kind of really disappointed," she told AFP at the Hong Kong airport on Tuesday morning.

Thousands of workers rushed home on Tuesday afternoon after the T8 signal was announced, with bus operators saying that service will be reduced later in the day.

Classes were to be suspended on Tuesday and Wednesday, while the Hong Kong Jockey Club also cancelled Wednesday's horse-racing meet.

Hong Kong's stock exchange changed its rules this year to keep markets open during typhoons, with the operator telling Bloomberg News that it was "closely monitoring" the situation.

Ragasa -- named after the Filipino word for rapid motion -- is expected to make landfall in the central and western coastal areas of Guangdong within 24 hours, the province's emergency management bureau said on Tuesday morning.

Shenzhen earlier ordered the evacuation of 400,000 people.

Emergency management authorities in the Chinese tech hub said that except for emergency rescue personnel and those ensuring people's livelihood, "do not go out casually".

Other cities in the Southern province of Guangdong that are implementing the measures include Chaozhou, Zhuhai, Dongguan and Foshan.

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