Tsunami waves have hit Hawaii and swept through the island chain after an earthquake in Japan sparked evacuations throughout the Pacific and as far as the US western coast.
The Pacific Tsunami Warning Centre said Kauai was the first of the Hawaiian islands hit by the tsunami.
Water rushed ashore in Honolulu, swamping the beach in Waikiki and surging over the break wall in the world-famous resort but stopping short of the area's high-rise hotels.
Waves about six feet (two metres) high were recorded on Maui, and three feet (a metre) in Oahu and Kauai.
Officials warned that the waves would continue and could become larger, but a scientist at the tsunami warning centre said it did not appear that they would cause major damage in Hawaii.
"But there is going to be some damage, I'm sure," said geophysicist Gerard Fryer in Hawaii.
Roads and beaches were empty as the tsunami struck the state, which had hours to prepare.
Sirens sounded throughout the night, and residents in coastal areas were sent to refuge areas at community centres and schools while tourists were moved to higher floors of hotels.
People waited in long lines stocking up on gas, bottled water, canned food and generators.
The tsunami, spawned by an 8.9-magnitude earthquake in Japan, raced across the Pacific at 500 mph - as fast as a jetliner - and could keep that speed until it hits a large land mass, though the waves roll into shore at normal speeds.
Waves are predicted to hit the western coast of the United States between 4pm and 4.30pm Irish time today.
Evacuations were ordered in parts of Washington and Oregon, and fishermen in Crescent City, California, fired up their crab boats and left the harbour to ride out an expected swell. A tsunami in 1964 killed 11 people in Crescent City.
It was the second time in a little more than a year that Hawaii and the US West coast faced the threat of a massive tsunami.
A magnitude-8.8 earthquake in Chile spawned warnings on February 27, 2010, but the waves were much smaller than predicted and almost no damage was reported.
Scientists acknowledged they overstated the threat but defended their actions, saying they took the proper steps and learned the lessons of the 2004 Indonesian tsunami that killed thousands of people who did not get enough warning.
Today Honolulu International Airport remained open but seven or eight jets bound for Hawaii have turned around, including some originating from Japan.
All harbours are closed.
Many islands in the Pacific evacuated after the warnings were issued, but officials told residents to go home because the waves were not as bad as expected.
But the size of Hawaii's islands is expected to amplify the waves, which will crash hardest against harbours and inlets.
"They're going to be coming in with high currents, they can pick up boulders from the sea floor ... they can pick up cars, they can pick up fuel tanks, those things become battering rams and so it just amplifies the destruction in a big tsunami," said Chip McCreery, director for the Pacific Tsunami Warning Centre.
Waves almost five feet (1.5 metres) high hit Midway, a tiny island in the North Pacific about 1,300 miles north-west of Honolulu.
The warnings issued by the Hawaii-based Pacific Tsunami Warning Centre cover an area stretching the entire western coast of the United States and Canada from the Mexican border to Chignik Bay in Alaska.
In the Canadian pacific coast province of British Columbia, authorities evacuated marinas, beaches and other areas.
In Alaska, a dozen small communities along the Aleutian Island chain were on alert, but there were no reports of damage from a wave just over 5 feet.
Officials in two coastal Washington counties used an automated phone alert system, phoning residents on the coast and in low-lying areas and asking them to move to higher ground.
In Oregon, sirens blasted in some coastal communities and at least one hotel was evacuated in the northern part of the state.
Latin American governments ordered islanders and coastal residents to head for higher ground.
First affected would be Chile's Easter Island, in the remote South Pacific, about 2,175 miles east of the capital of Santiago, where people planned to evacuate the only town.
Ecuador's President Rafael Correa declared a state of emergency and ordered people on the Galapagos Islands and the coast of the mainland to seek higher ground.