Hundreds of people in Taiwan have been evacuated from their homes, and many schools and offices closed, as the biggest typhoon in decades sweeps towards the region on Friday.

Typhoon Bavi is expected to pound Taiwan's north and east, as well as Japan's remote southwestern islands, on Friday and Saturday before smashing into China, which has been hit by deadly storms this week.

Locals in the port city of Keelung, which is expected to be one of the hardest hit areas, stocked up on food, taped windows, and stacked sandbags at the entrances to street-level businesses, heeding warnings from authorities to take precautions.

"We're worried," said Samuel Fu, who works in a noodle shop in the coastal district of Bali, near Taipei.

"This is the first time we've experienced such a big typhoon... since we opened the shop," 20-year-old Fu told AFP.

"The signboard and the awning at the front could be blown off. If the winds get stronger, we'll bring everything inside."

After hitting Guam and the Northern Marianas on Monday as a super typhoon, Bavi was downgraded to a typhoon as it moved across the Pacific Ocean.

Bavi's maximum sustained wind speeds were 162 kilometres (100 miles) per hour, with gusts of around 198 kilometres per hour, on Friday, slower than on Thursday, the Central Weather Administration (CWA) said.

"The typhoon is likely to continue weakening because environmental conditions are not favourable," CWA forecaster Wang Ping-hsiang said.

"The greatest impact is expected in Taipei, New Taipei, Keelung and Yilan, while the heaviest rainfall is forecast for mountainous areas in central and northern Taiwan."

Bavi's strong-wind radius of 380 kilometres (240 miles) will make it the largest typhoon to hit Taiwan in more than 30 years.

Many schools and offices were shuttered on Friday across northern and eastern Taiwan, including Taipei.

In Keelung, locals flocked to a fresh food market to buy fruit and vegetables, street food vendors secured their stands, and a temple covered and tied down an outdoor statue ahead of the storm.

Bavi is expected to dump up to a metre of rain, raising concerns about potential flooding and landslides.

More than a thousand people have been evacuated from their homes, mostly in the mountainous county of Hualien in Taiwan's east where authorities are monitoring two barrier dams.

Taiwanese President Lai Ching-te urged people in areas likely to be most affected by Bavi to remain on "high alert"

"Although the typhoon has weakened slightly and is now classified as a moderate typhoon, its extensive wind field may still bring strong winds and heavy rain to various areas," Lai said on Facebook.

After sweeping past Taiwan, Bavi is expected to make landfall in eastern China over the weekend.

Extreme weather has already wreaked havoc on southern and central China this week, with storms leaving at least 39 dead and causing dozens of rivers to overflow and a reservoir dam to burst.