Mangkhut weakened from a typhoon to a tropical storm as it moved deeper into southern China on Monday, leaving a trail of death and destruction from Hong Kong to the Philippines.

The storm was still raging across southern China's coast and the provinces of Guangdong, Guangxi and Hainan and rain and strong winds were expected to continue through Tuesday.

Hong Kong residents were told to stay away from the coastline and be on alert for occasional gales.

Bus, ferry and rail services were suspended and almost 900 flights were cancelled at the city's airport, one of the world's busiest. The South China Morning Post said Hong Kong's hospitals had to use backup power due to outages caused by the storm.

Authorities reported four deaths from falling trees and building materials in Guangdong, China's manufacturing hub.

Mangkhut earlier lashed the Philippines, sparking landslides and building collapses that killed at least 65 people, with another 43 missing.