Footage circulating on social media shows fierce winds, tearing off roofs and parts of buildings in Hong Kong and nearby areas.
The storm, with gusts of more than 230 kilometres per hour (142 mph), sent buildings swaying and waters surging into homes and shopping malls in Hong Kong, with some roads waist-deep in water.
The typhoon prompted storm surges as high as 10ft. and flooded ground-floor buildings in Shenzhen.
In Macau, next door to Hong Kong, casinos were ordered to close from 11 p.m. Saturday, the first time such action was taken in the city, the South China Morning Post reported. Not only thus, the city remained entirely shut down.
Nearly all flights in and out of Hong Kong were canceled.
Mayor Mauricio Domogan said three people died and six others were missing in his mountain city of Baguio after several houses were destroyed.
Vietnam can breathe a sigh of relief as the massive storm, which has killed 64 people in the Philippines and two others in China, turned into a tropical depression at 10 a.m. on Monday after it made landfall on the coast of Jiangmen city, in China's Guangdong Province.
Lily Chiu, a 60-year-old caregiver in Hong Kong, had stayed overnight at the nursing home where she worked to avoid commuting to an early shift when the storm hit. More than 1,000 people sought refuge in 48 temporary shelters. And more than 2.4 million people were relocated, according to the AP.
The death toll from Typhoon Mangkhut in the Philippines has reached 28, a police director said Sunday, attributing the count mostly to landslides in the nation's northern provinces that were triggered by the storm.
"They were barricading doors with sandbags, tying the handles together with zip ties which was a little bit scary", she said.
Hong Kong's famed Victoria Harbor was hit with a storm surge of more than 3.9 meters (12.8 feet) above chart datum Sunday.
Mangkhut made landfall in the city of Taishan, in Guangdong, at 5pm, packing wind speeds of 162kph.
In the eastern Hong Kong neighbourhood of Heng Fa Chien, residents of a flooded housing complex linked arms and held hands as they waded knee-deep through pools of rainwater in streets, parking lots and stores.
"It was not so severe as we expected it to be because earlier it was noted it would also be strong", said President Rodrigo Duterte, following an aerial survey of some affected areas.
Transport services on Hong Kong Island have been suspended, as have ferries to mainland China.
Hong Kong's home affairs department said it had opened 48 temporary shelters and was now housing more than 1,200 people displaced by the storm.
Guangdong is also China's most populous province, with a population of more than 100 million.
The most severe damage came in Luzon's north, a sparsely populated region that's considered the breadbasket of the Philippines, though areas as far away as Manila - more than 340 km (200 miles) from the eye of the storm - were hit with heavy rains that caused flooding in urban areas.