The world's strongest storm this year, Super Typhoon Mangkhut, is blasting the northern coast of the Philippines with strong winds and heavy rain.
Reports say it broke windows and downed power lines on the island of Luzon before dawn.
More than four million people are directly in the path of the storm, which has winds of 200km/h (125 mph) and gusts reaching up to 330km/h.
Thousands have been evacuated amid warnings of 6m (20ft) storm surges.
Officials have warned of there may be "very heavy damage" to vulnerable structures.
They have raised alert levels to the second-highest - signal four - in Cagayan, northern Isabela, Apayao and Abra provinces.
The deadliest storm on record in the Philippines was Super Typhoon Haiyan in 2013, which also sparked category four alerts.
It killed more than 7,000 people and affected millions.
What is the latest?
The typhoon made landfall at Baggao, in the north-east of the country, at about 01:40 local time on Saturday (17:40 GMT on Friday).
The storm - known locally as Ompong - is travelling north-west at about 35km/h (20 mph).
"In terms of strength, Typhoon Mangkhut is the strongest tropical cyclone of the year," the World Meteorological Organization said.
The typhoon is forecast to keep heading west, passing through Hong Kong on Sunday afternoon.
Hong Kong authorities have warned urged residents to stay indoors when the storm approaches, and weather experts say it may be the strongest tropical storm to hit the territory in decades.
Hong Kong authorities have warned urged residents to stay indoors when the storm approaches.
The typhoon is expected to weaken into a tropical depression by Tuesday.
How prepared are people?
Warnings have been issued in dozens of provinces, and sea and air travel has been restricted.
Flights have been cancelled, schools shut and the army is on standby.
The authorities have also warned that heavy rains could trigger landslides and flash floods.
"We are really frightened," Delaila Pasion, who had fled her home, told AFP news agency. "They say it is so strong, we were too scared to remain."
"During previous monsoon rains, half of our house was destroyed so I wanted to take my grandchildren to safety," she told journalists.
The Philippines is routinely hit during the typhoon season.
In China, where the storm is predicted to hit late on Sunday or early on Monday, the authorities have raised storm alerts to "yellow", which is the second level on the four-tier warning system.
High-speed rail services have been cancelled in parts of the south, local media report.
Is global warming to blame?
The relationship between climate change and tropical storms is a complex one.
Typhoons and hurricanes form when air, heated by warm sea water, rises quickly.
As the air cools down again it is pushed aside by more warm air rising below it - causing strong winds and whipping up waves.
So as the temperature of ocean water goes up, we might expect the intensity of hurricanes to increase in future.
A hotter atmosphere can also hold more water, so this should allow hurricanes to dump more water on affected areas.
But there are so many factors that contribute to these events, it has been difficult to tease out clear trends from the data.
Calm before the storm
Howard Johnson, BBC News, Arparri, Luzon
Residents of this small, low-rise town say they've been through this before, they've seen other storms.
Farmers are working around the clock to harvest their rice paddies. They say the rice is still not ripe, but want to salvage what they can before this storm potentially devastates their fields.
Authorities have told people to move on. Some cars are leaving the area, bedding on the roof - but some people say they want to stay inside their homes to prevent thieves entering.
People seem to be very relaxed with the idea that a super typhoon is coming their way.