Residents hunkered down in their apartments, and streets in the usually buzzing city were deserted Sunday.
Hong Kong Security Minister John Lee Ka-chiu urged residents to prepare for the worst.
The Philippines' deadliest storm on record is Super Typhoon Haiyan, which left more than 7,350 people dead or missing across the central part of the country in November 2013.
A park is flooded by seawater after Typhoon Mangkhut hit Hong Kong, China, Sept. 17, 2018. "More than two million people have been evacuated in China's Guangdong province, and almost 20,000 emergency shelters have been activated".
Mangkhut has swept over Hong Kong and Macau and on into China's southeast province of Guangdong, after devastating the Philippines, where the toll is likely to surpass 50.
The paper said the heavy rains brought storm surges of 3 meters (10 feet) around Hong Kong. Authorities there raised the storm warning signal to T10 earlier, the highest possible level.
Meanwhile, 391 people, including 219 men and 172 women, have sought medical treatment at the accident and emergency services wards at public hospitals during the typhoon period.
The typhoon weakened to a tropical storm Monday morning as it moved deeper into southern China, where rain and strong winds were expected to continue through Tuesday. Hundreds of flights were canceled at Hong Kong International Airport, a major transportation hub for the region.
The storm shattered glass windows on commercial skyscrapers in Hong Kong, sending sheets of paper pouring out of the buildings. In the Philippines, where most of the deaths have occurred, a landslide caused by the storm is mainly to blame.
"They laughed at our policemen", he said.
Francis Tolentino, a political adviser to President Rodrigo Duterte, told the BBC he estimated only a fifth of produce there had been harvested in advance - threatening staples like rice and corn.
In Macau, next door to Hong Kong, casinos were ordered to close from 11:00pm Saturday, the first time such action was taken in the city. About 63,000 people have been affected, and the damage has not yet been fully assessed as numerous worst-hit areas are rural. Phone lines and communication in parts of the province have also been affected.
Authorities said they believed many of those buried in the landslide were gold miners who'd been working illegally at a mine formerly operated by Benguet Corp., a Philippine mining firm.
People who had crowded into evacuation centres were receiving government food packages, Ms Villamar said, and water was being delivered as the mains supply had been compromised.
The state broadcaster also said flood warnings had been issued for 38 rivers in the neighboring region of Guangxi, while 12 coastal monitoring stations reported their biggest-ever waves.
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.