If the typhoon remains at No. 8 or higher, Hong Kong's stock trading will be canceled Monday.
Mangkhut had shut down the city all day Sunday after the Observatory gave the storm its highest warning signal. Hainan Airlines cancelled 234 flights in the cities of Haikou, Sanya, Guangzhou, Shenzhen and Zhuhai this weekend.
Philippine authorities said a baby and a toddler were among the 29 dead, a lot of them in landslides in mountainous areas that left at least 13 missing.
Mangkhut is now expected to move inland of western Guangdong.
A girl walks on debris caused by Typhoon Mangkhut on the Hong Kong waterfront.
Secretary for Security John Lee Ka-chiu described the damage as "serious and extensive" with the number of calls for help, or reports of injury, five times higher than in August past year when Super Typhoon Hato battered Hong Kong.
Mayor Victorio Palangdan of Itogon town in Benguet province, among the worst-hit by the typhoon that struck Saturday, said at a news conference that of the 40 to 50 people thought buried, there's a "99 per cent (chance) that they really are all dead". Dozens of people rushed into a three-storey building that was later destroyed by a landslide.More news: PSG coach Tuchel: Liverpool geniune Champions League contenders
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The 19 who died were part of a bigger group of 43 people, likely miners, and those who were still alive were feared to be trapped in an old mining bunkhouse that had collapsed under rubble, Salamat said. Three villagers who managed to escape told authorities what happened.
The victims had sought refuge in a church during the typhoon - but Lt Gen Salamat said they were told to flee the town.
Palangdan said three villagers who almost 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. Police and their vehicles could not immediately reach the landside-hit area because the ground was unstable and soaked from the heavy rains, regional police chief Rolando Nana told the ABS-CBN TV network.
The typhoon has weakened to a tropical storm.
Videos posted by residents showed the incredible power of the typhoon as it caused buildings to sway, sucked reams of documents out of the shattered office windows, tore down scaffolding and pushed walls of water into low-lying parts of the island.
Mangkhut made landfall in the Guangdong city of Taishan at 5:00pm local time, packing wind speeds of 162 kilometres per hour. State television broadcaster CGTN reported that surging waves flooded a seaside hotel in the city of Shenzhen.
10 - the strongest alert under Hong Kong's storm warning system - was in force for about ten hours on Sunday after it was raised shortly before 10 am, bringing the city to a complete shutdown.
Along the coast, the gambling enclave of Macau closed all its casinos, and all fishing boats from China's Guangdong province were called into port.