The typhoon was packing sustained winds of 205 kilometer per hour and gusts of up to 255 kph when it smacked the province.
At that time, the center of the depression was around 200 kilometers (124 miles) from the Vietnam-China border with wind speeds of up to 60 kilometers per hour.
Despite avoiding a direct hit, winds there reached more than 110mph and as water levels surged by nearly 3.5m (12ft) in places.
A resident living in a high-rise in the city told Reuters news agency she could feel her building swaying in the storm. "I've not seen the roads flood like this, (and) the windows shake like this, before".
Hundreds of flights were cancelled.
"The landslides happened as some residents returned to their homes after the typhoon", disaster response coordinator Francis Tolentino told DZMM Radio, adding that most of the 5.7 million people affected had made advance preparations.
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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. Eight more people are missing there. Mangkhut is blowing from the Pacific and forecast to directly slam the province's northeastern coastal and island municipalities, Mamba said. Rail, airline and ferry services were restored and casinos in the gambling enclave of Macau reopened.
Ports, oil refineries and industrial plants in Guangdong have been shut.
Office of Civil Defense chief Ricardo Jalad said other northern provinces will also start evacuating residents from low-lying areas on Thursday. In Shenzhen, electricity supply to more than 130,000 homes was cut at one point on Sunday. Guangdong produces about 1 million tonnes, or a tenth of national sugar output.
The storm has fueled concern about Guangdong's sugar output, with China sugar futures rising last week on fears for the cane crop.
"More than two million people have been evacuated in China's Guangdong province, and almost 20,000 emergency shelters have been activated".
Work has been suspended at more than 29,000 construction sites and 640 tourist spots were closed.
State television showed scenes of crashing waves, innundated streets and trees half-bent by the strong winds as Mangkhut unleashed its power.
In China, all flights at Guangzhou International Airport were canceled from midday Sunday through Monday morning.