Residents in coastal areas across the Pacific from Hawaii to Guam were ordered to evacuate to shelters and higher ground.
In Hawaii's tourist district of Waikiki, visitors were being moved to the higher floors of their hotels. Residents waited in long lines, stocking up on gas, bottled water, canned food and generators.
"We're preparing for the worst and we're praying for the best," said John Cummings III, spokesman for the Honolulu Department of Emergency Management.
Tsunamis can travel at speeds of 800kmh, as fast as a jetliner.
Waves could hit the western coast of the United States between 7am and 7.30am (Pacific Standard Time) on Friday.
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 Alaska, a dozen small communities along the Aleutian Island chain were on alert.
"Everyone in that area knows, when you feel it, move - don't wait for a siren," said John Madden, director of the Alaska Division of Homeland Security and Emergency Management.
The largest affected town is Unalaska, with a population about 4000.
The tsunami was expected to hit the Northern Mariana Islands, a US territory, at 1am PST, but no big waves came.
Waves about 80cm high hit the beach in Saipan, and sirens still sounded in the empty streets.
Maria Mettao, who works at the front desk of the Hyatt Regency Saipan in the Northern Marianas, said hotel staff has been given the all-clear.
Mettao said the hotel has allowed guests to leave from the higher floors where they had been evacuated.
In the Philippines, officials ordered an evacuation of coastal communities along the country's eastern seaboard in expectation of a tsunami following the 8.9-magnitude earthquake in Japan.
Disaster management officials in the Albay province southeast of Manila say they ordered residents to move to designated evacuation sites that are at least 5m above sea level.
In Guam, authorities advised people to evacuate low areas of the US territory and seek ground higher than 17m above sea level and 34m inland.
Australia was not in danger because it was protected by island nations to the north, including Papua New Guinea and the Solomon Islands, that would largely absorb any wave activity, said Chris Ryan, a forecaster at the National Meteorological and Oceanographic Centre, the Australian government agency that monitors the threat.
Honolulu's Department of Emergency Management has created refuge areas at community centres and schools, and authorities on Kauai island have opened 11 schools to serve as shelters for those who have left tsunami innundation zones.
Gov. Neil Abercrombie told residents to take the warning seriously and head for higher ground.
A small 4.5-magnitude earthquake struck the Big Island just before 2am, but there were no reports of damages and the quakes weren't likely related, a geophysicist with the United States Geological Survey said.
US Coast Guard rescue crews were making preparations throughout the Hawaiian Islands to provide post-tsunami support, with cutter and aircraft crews positioning themselves to conduct response and survey missions.
Dennis Fujimoto said the mood was calm but concerned on the island of Kauai while people readying for the tsunami.
There's long lines at gas stations, and at the Wal-Mart, one of the few places that was open to midnight, people were stocking up on supplies.
"You got people walking out of there with wagonloads of water," he said.
Chip McCreary, the centre's director, said tsunami waves have the potential to swamp coastal areas of all Hawaii's islands.
"What these waves look like is an elevation of sea level, where the sea level will rise above its normal level and stay high for 10 or 15 minutes before it starts to recede," he said.
"As a result of this, in a tsunami wave, that water can flood the coast line and be a hazard to people and buildings on the coast."
McCreary said the threat will become clearer when the waves hit Wake Island and Midway.
"Tsunami waves, because of their long length, they wrap around our islands very efficiently," he said.
Readings have come in from deep ocean gages deployed since the 2004 tsunami in Banda Acha in Japan and around Wake Island.
