The storm also broke windows, felled trees, tore bamboo scaffolding off buildings under construction and flooded areas with sometimes waist-high waters, according to the South China Morning Post.
The paper said the heavy rains brought storm surges of 3 meters (10 feet) around Hong Kong.
Hong Kong Security Minister John Lee Ka-chiu urged residents to prepare for the worst.
"Because Mangkhut will bring winds and rains of extraordinary speeds, scope and severity, our preparation and response efforts will be greater than in the past," Lee said. "Each department must have a sense of crisis, make a comprehensive assessment and plan, and prepare for the worst."
Hong Kong's Cathay Pacific said all of its flights would be canceled between 2:30 a.m. Sunday and 4 a.m. Monday. The city of Shenzhen also canceled all flights between Sunday and early Monday morning. Hainan Airlines canceled 234 flights in the cities of Haikou, Sanya, Guangzhou, Shenzhen and Zhuhai scheduled this weekend.
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. Macau suffered catastrophic flooding during Typhoon Hato last year, leading to accusations of corruption and incompetence at its meteorological office.
In Macau's inner harbor district on Sunday, the water level reached 1.5 meters (5 feet) and was expected to rise further. The district was one of the most affected by floods from Typhoon Hato, which left 10 people dead.
In the northern Philippines, about 40 to 50 people, mostly small-scale miners and their families, were feared to have been trapped by a landslide in the far-flung village of Ucab in Itogon town in Benguet province, Itogon Mayor Victorio Palangdan said by phone.
Palangdan said three villagers who nearly got buried by the huge pile of mud and rocks told authorities they saw residents rush into an old three-story building, a former mining bunkhouse that has been transformed into a chapel, at the height of the typhoon's onslaught Saturday afternoon.
"That was not an authorized evacuation center," Palangdan said, but expressed sadness that the villagers, many of them poor miners, had few options to survive in a region where big corporations have profited immensely from gold mines.
Police Superintendent Pelita Tacio said rescuers pulled out seven bodies from the huge mound of mud and rocks. She said they were scrambling to pull out the body of another victim before she left the area Sunday.
"It's very sad. I could hear villagers wailing in their homes near the site of the accident," Tacio said by phone.
Rescuers stopped their search intermittently because of heavy rain, and the search and rescue operation was suspended at nightfall and will resume at daybreak Monday, Palangdan said.
Overall, at least 36 people have died in typhoon-related incidents in the Philippines, mostly from landslides and collapsed houses.
Mangkhut made landfall on Saturday on the northeastern tip of Luzon island in the Philippines with sustained winds of 205 kilometers (127 mph) and gusts of 255 kph (158 mph).
The Philippines appeared to have been spared the high number of casualties many had feared. In 2013, Typhoon Haiyan left more than 7,300 people dead or missing, flattened villages and displaced more than 5 million in the central Philippines. A massive evacuation ahead of Mangkhut helped lessen potential casualties, with about 87,000 people evacuating from high-risk areas, officials said.
Philippine National Police Director General Oscar Albayalde told The Associated Press that 20 people died in the Cordillera mountain region, four in nearby Nueva Vizcaya province and another outside of the two regions. Three more deaths were reported in northeastern Cagayan province, where the typhoon made landfall.
The typhoon struck at the start of the rice and corn harvesting season in the Philippines' northern breadbasket, prompting farmers to scramble to save what they could of their crops, Cagayan Gov. Manuel Mamba said.