Tsunami waves began rolling into Hawaii early Friday, with readings of between 6 and 8 feet expected in some areas after an earthquake struck Japan.
Communities along much of the U.S. West Coast were under tsunami warnings, too.
The first impact in Hawaii was felt shortly after 3:07 a.m. local time (8:07 a.m. EST), according to Hawaii State Civil Defense, which issued a tsunami warning.
The Pacific Tsunami Warning Center said Kauai was the first island the tsunami hit. Water rushed ashore in Honolulu, swamping the beach in Waikiki and surging over the breakwall in the world-famous resort, but stopping short of the area's high-rise hotels.
Waves at least 3 feet high were recorded on Oahu and Kauai and officials warned that the waves would continue and could become larger. Roads and beaches were empty as the tsunami struck the state, which had hours to prepare.
The National Weather Service said the waves could hit Oregon and California between 7:15 a.m. and 7:30 a.m. local time (10:15-10:30 a.m. EST).
The threat of a tsunami prompted the U.S. National Weather Service to issue a warning for at least 50 countries or territories around the Pacific Ocean after the 8.9-magnitude earthquake struck Japan on Friday.
Warnings also were in effect for coastal areas of California and Oregon from Point Concepcion, California, to the Oregon-Washington border, according to the U.S. Pacific Tsunami Warning Center.
A warning also was in effect also for Alaska, from Amchitka Pass to Attu, and in Canada's British Columbia.
President Barack Obama said he instructed the Federal Emergency Management Agency to be prepared to help Hawaii and other U.S. regions "that could be affected" by the disaster.
According to media reports, hundreds are dead after the worst earthquake in generations struck off the northeast coast of Japan, setting off a devastating tsunami that swallowed swaths of coastal territory and fanned out across the Pacific.
The 8.9-magnitude earthquake - the world's fifth-largest since 1900 and the biggest in Japan in 140 years - struck at 2:46 p.m. local time, shaking buildings violently in Tokyo for several minutes and sending millions fleeing for higher ground.