The quake was felt strongly in Tainan, a city of two million people and many were sleeping when buildings started falling down.
Hundreds of people are feared trapped in collapsed buildings after a 6.4 magnitude earthquake struck Taiwan a few minutes ago.
'Multiple' buildings - including at least one residential block - are said to have fallen in the city in Tainan following the quake which shook the island at about 4am on Saturday.
Pictures show buildings partially or completely destroyed, while video footage shows rescue workers wading through water after the mains burst.
At least five aftershocks of 3.8-magnitude or more have shook the city about half an hour after the initial quake.
The initial quake was very shallow, at depth of 6.2 miles (10 km), which would have amplified its effects, the United States Geographical Survey said in a statement.
More soon.