The most powerful storm to hit the Caribbean islands of the Bahamas since records began has torn roofs from buildings and caused severe flooding.
The slow-moving, category five Dorian – the second-strongest Atlantic hurricane on record – has sustained winds of up to 180mph (285km/h).
A "life-threatening" storm surge of 23ft (7m) is predicted in places.
The hurricane is moving slowly westwards and may hit areas of the eastern US seaboard.
The US states of Florida, Georgia, North and South Carolina have all declared states of emergency.
There is no word so far on any casualties.
What's the latest from the Bahamas?
The National Hurricane Center said that at 03:00 GMT, Dorian was located on the eastern edge of Grand Bahama, having made landfall earlier on the Abaco islands, which are just to the east.
Both chains are in north of the Bahamas archipelago. Grand Bahama is only 100km (60 miles) east of West Palm Beach in Florida.
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Dorian is slow-moving, travelling west at only 9km/h (6mph).
There was little information overnight from the affected Bahamas islands amid power cuts and limited internet access. The Abaco islands were reported under water after the storm hit on Sunday, with a storm surge of up to 7m (23ft).
Government officials said they had seen footage from Abaco of floodwaters the height of a car roof, and homes without roofs.
High winds and torrential rainfall have also struck Grand Bahama.
Residents posted footage showing floodwaters engulfing some homes after high winds had struck. Videos also show capsized boats floating in floodwaters filled with debris./BBC news