Storm Noru was 88 km from Da Nang at midnight of Tuesday, carrying maximum winds of 149 kph. It weakened to 133 kph more than an hour later as it hit the coast.
Earlier, heavy winds triggered by the storm had caused around 300 houses in the central province of Quang Tri to collapse, injuring eight people.
Torrential rains were recorded from 7 a.m. to 10 p.m. on Tuesday in Thua Thien Hue (280 mm), Da Nang (230 mm) and Ly Son Island off Quang Ngai Province (240 mm).
At 2 a.m. Wednesday, residents in Da Nang and Quang Nam reported "terrifying" winds as metal roofs were blown off despite being secured earlier.
Power cut was recorded on large scale in Da Nang and Thua Thien Hue. It also affected 210,000 families in Quang Ngai, more than 345,000 families in Quang Nam, including parts of tourist town Hoi An, which was deserted on Wednesday morning.
Strong waves and high tides flooded several coastal roads in Da Nang.
By 7 a.m. Wednesday, the storm has weakened to 74 kph. It is expected to descend into a tropical depression and a low pressure zone over Thailand.
Residents and the authorities have started to clean up the damage left by the storm. They are bracing for heavy rains forecast on Wednesday and Thursday.