China's National Meteorological Center issued an alert saying Mangkhut would make landfall somewhere on the coast in Guangdong province on Sunday afternoon or evening.
Rain and powerful winds are forecast to continue through Tuesday along China's coast and the provinces of Guangdong, Guangxi, and Hainan.
The storm weakened considerably after its powerful assault on the Philippines' island of Luzon, where at least 28 have died and more than 150,000 were displaced. Tens of thousands of small-time miners in recent years have come to the mountain provinces from the lowlands and established communities in high-risk areas like the mountain foothills of Itogon.
Dozens of people believed buried in a landslide unleashed by Typhoon Mangkhut in the Philippines probably did not survive, a mayor said Monday, although rescuers kept digging through mud and debris covering a chapel where they had taken shelter.
Mangkhut, the Thai name for the mangosteen fruit of southeast Asia, was expected to skirt around 100 kilometres south of Hong Kong and veer westwards towards the Chinese coastline of Guangdong province, as well as the gambling hub of Macau straddling the Pearl River Delta.
Numerous missing in the Philippines are gold miners and their families feared buried in a landslide after seeking shelter in a bunkhouse-turned-chapel in a village in Benguet province.
Hundreds of flights from the city's airports were delayed or cancelled, and much of the city's public transport has been suspended.
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Ferry services in the Qiongzhou Strait in southern China were halted on Saturday and helicopters and tugboats were dispatched to Guangdong to transfer offshore workers to safety and warn ships about the typhoon, China's official Xinhua News Agency reported.
The Hong Kong government said Mangkhut will pose "a severe threat to the region" as many residents in the city and neighbouring Macau stocked up on food and supplies. Many of the Philippines' deaths were caused by landslides.
On Sunday morning, the typhoon featured sustained winds of 96 miles per hour and gusts of up to 118 mph.
Mangkhut battered the northern Philippines on Saturday before slamming into southern China on Sunday.
Landslides caused by the pounding storm hit two villages in Itogon town in the Philippine mountain province of Benguet.
The plans of tens of thousands of travellers were disrupted by flight cancellations at Hong Kong's global airport, a major regional hub.
Mangkhut earlier lashed the Philippines, sparking landslides and building collapses that killed at least 64 people.