Boats were thrown onto the shore by powerful waves and the huge rainfall has brought fears of landslides, although none have yet been reported in Hong Kong.
Authorities in southern China had issued a red alert, the most severe warning, as the national meteorological center said the densely populated region would face a "severe test caused by wind and rain" and urged officials to prepare for possible disasters.
Hong Kong raised its highest No. 10 typhoon signal at mid-morning as ferocious winds uprooted trees and smashed windows in office and residential buildings, some of which swayed in the gusts, residents said. All high-speed and some normal rail services in Guangdong and Hainan provinces were also halted.
Videos and images coming in from Hong Kong and nearby Shenzhen, China, show how destructive the storm is.
The monumental task of cleaning up the city began as residents, some in suits and ties, struggled to get back to work on roads that remained blocked by felled trees, mud and debris.
Mangkhut was still expected to bring heavy rain to Hong Kong, with flood warnings in place for low-lying areas.
Mangkhut is expected to be one for Hong Kong's record books.
More than 550 flights have been canceled at airports in Hong Kong, Shenzhen and Guangzhou, and more than 200 have been delayed, according to Flightaware.com.
Shenzhen also canceled all flights between yesterday and early this morning.
The cities are nearly entirely shut down and one of Macau's main attractions, local casinos, were ordered to close for the first time in history, CGTN reports. Macau suffered catastrophic flooding during Typhoon Hato a year ago, leading to accusations of corruption and incompetence at its meteorological office.
The city's famed Victoria Harbor is expected to see a storm surge of 3.5 meters later Sunday (11.5 feet).
He condoled with the families of those killed in the typhoon.
The death toll in the Philippines, where the main island of Luzon was mauled with fierce winds and rain, rose to 65 overnight as rescuers pulled more bodies from a ill-prepared landslide in the mountain town of Itogon.
The bunkhouse was a shelter at an old mining site in the town of Itogon, almost 200km north of Manila, which "went missing" during a landslide, said Itogon Mayor Victorio Palangdan.
Police Superintendent Pelita Tacio said rescuers pulled out seven bodies from the huge mound of mud and rocks on September 16-She said they were scrambling to pull out the body of another victim before she left the area.
While the death toll is now 54, with 32 people injured and 42 more missing, this is likely to rise in the coming days as rescue teams work to locate 36 people believed to be buried beneath landslides in the Cordillera Administrative Region, in the north of Luzon island.
Mangkhut earlier lashed the Philippines, sparking landslides and building collapses that killed at least 65 people, with another 43 missing.
Presidential adviser Francis Tolentino said about 87,000 people had been evacuated from high-risk areas and advised not to return home until the danger has passed.
Three more deaths have been reported in northeastern Cagayan province, where the typhoon made landfall.
"The Zhuhai city government spent the financial subsidy that they received from the central government past year on rebuilding the city", paving new roads and reinforcing sidewalks, he said.