Typhoon Tembin swept across southern Taiwan Friday, toppling trees and ripping off rooftops after thousands of people were evacuated to avoid a repeat of a deadly storm three years ago.

Tembin, characterised as a "severe" typhoon by the Hong Kong Observatory, hit before dawn and ground its way across Taiwan's southern tip before moving out to sea again, where it was still lingering after dark.

The typhoon left five people injured in its path, according to Taiwan's Central Emergency Operation Centre.

In Kaohsiung city in the south, a firefighter was electrocuted while handling a power cable ripped off by powerful winds, receiving burns so serious that his right foot may need to be amputated, the centre said.

Another firefighter also received burns in the same accident but was in stable condition after medical treatment, the centre said, adding that three other people were injured by broken glass in Taitung county in the east.

Fallen tree trunks covered roads in the south, rocks loosened by torrential rain dropped from mountainsides, and in the coastal village of Tawu a number of rooftops were blown off by strong winds.

In towns along the rugged east coast, thousands of people were evacuated and schools and businesses shut down as communities braced for the typhoon to hit, but Tembin made landfall further south than expected. Island-wide, the number of evacuees topped 7,000.

Even though the casualties were fewer than feared, farmers said their livelihoods had been badly affected by the powerful weather system.

"This is more serious than we thought it would be," said Chen Yue-zi, a fruit and nut grower near the eastern city of Taitung, as he looked at his orchard, littered with trees toppled by heavy winds overnight.

The Central Weather Bureau said the typhoon had lost strength and was downgraded to a tropical storm after it passed over Taiwan's mountainous southern terrain and moved out to sea on a looping path.

As of 6:15 pm (1015 GMT), it was about 50 kilometers (31 miles) southwest of Kaohsiung, according to the Central Weather Bureau, which warned it could circle back.

"Today and tomorrow will be a crucial time. It may come back. It's determined by a multitude of factors," said weather bureau forecaster Lo Ya-yin.

As the storm bore down, police set up roadblocks in several locations leading to areas considered in danger of landslides, only letting through those on urgent business.

"It could have been worse if we hadn't closed parts of the highway last night," said an official with the highway administration, who gave his name as Wang.

The typhoon dumped nearly 50 centimetres (20 inches) of rain in Pingtung county overnight, leaving four towns flooded in knee-deep brown water.

Television images showed rescuers and soldiers navigating through empty streets in rubber boats, while heavy-duty military trucks ploughed through the water to rescue trapped residents.

Strong winds also toppled a line of electricity poles in the county, cutting off power to tens of thousands of homes.

Across the island, more than 50,000 households were without power, the state Taiwan Power Company said.

According to the transport ministry, 61 domestic and 20 international flights were cancelled.

The high-speed rail service linking Taipei to Kaohsiung city in the south cut its services to three trains per hour, down from up to six per hour. The service resumed normal operation from 0800 GMT.

The evacuations were ordered in risk areas along Taiwan's coastline as authorities attempt to prevent a repeat of an enormously destructive typhoon which hit three years ago.

Typhoon Morakot killed about 600 people in August 2009, most of them buried in huge landslides in the south.

A senior military official told AFP Thursday that authorities had learned lessons from Morakot, when they "had not done enough evacuations beforehand".

Taiwan's military had ordered 50,000 soldiers to be on standby, ready to move out and assist in disaster prevention efforts.