The typhoon battered the Batanes island group in the far north of the Philippines overnight with gusts of up to 250 kilometres per hour, affecting communication lines and damaging crops, officials said.
"The winds are very strong. I cannot even go out now," Batanes governor Vicente Gato told DZBB radio in Manila. "Many trees have been uprooted and we have no electricity," he said.
The National Disaster Risk Reduction and Management Center issued its highest alert, with flooding recorded in four regions of the main island of Luzon, the country's most populous area, while several roads and bridges were rendered impassable by overflowing rivers or landslides.
In Taiwan, some flights were cancelled and ferry services suspended, with schools and offices in many parts of the island closed, especially in the south and east, which were expected to bear the brunt of the storm, authorities said.
Hotels and resorts in mountainous areas were closed due to fears of flooding and landslides.
Coastguards cordoned off the beaches at Kenting, a popular scenic spot in the south, as strong winds whipped up the sea.
Nearly 2,500 people had already been evacuated, officials said, as the Central Weather Bureau warned people to expect up to 1.2 metres of rain.
The US Navy's Joint Typhoon Warning Center said that Usagi was packing sustained winds of 240 kilometres per hour with gusts of nearly 300 kilometres per hour, making it the equivalent of a strong category four Atlantic hurricane.
As of 0400 GMT on today, Usagi was still roaring through the Luzon Strait separating the Philippines and Taiwan, heading directly for the southern Chinese coast.
"The weather will deteriorate significantly with high winds and rough seas," the city's observatory said.
