Hawaii has been hit by a tsunami, prompting the authorities to order at least 100,000 people on the island to move to higher ground.

The Pacific Tsunami Warning Centre said the first wave was feet high and less forceful than expected. Some forecasts had predicted a wave of up to six feet high.

"The tsunami arrived about when we expected it should," senior geophysicist Gerard Fryer told reporters at a news conference, saying: "I was expecting it to be a little bigger." Other waves were expected.

The tsunami hit with little warning and an alert, issued at short notice due to initial confusion among scientists about the quake's undersea epicentre, caused massive traffic congestion as motorists made a mass exodus from low-lying areas.

The Warning Centre had said the first tsunami wave would strike the islands at 10:28pm local time.

The mayor of Honolulu mayor, Peter Carlisle, announced that all police and emergency personnel were being withdrawn from potential flood zones shortly before the first wave, leaving anyone defying evacuation orders to fend for themselves. He urged motorists who remained caught in harm's way due to gridlocked roads to abandon their vehicles and proceed on foot.

"If you are stuck in traffic, you might consider getting out of your car and consider walking to higher ground. You will have to assess your own situation, depending on where you are right now. Right now it is critical," he said.

Vindell Hsu, a geophysicist at the Tsunami Warning Centre said an estimated 100,000-150,000 people who live in Hawaii's coastal zones had been urged to move to higher ground .

Governor Neil Abercrombie issued an emergency proclamation for the state.

The Tsunami Centre cautioned that the height of the waves could not be predicted and that the first wave "may not be the largest". It said: "All shores are at risk no matter which direction they face".

The warnings followed a powerful earthquake with a magnitude of 7.7 that hit Canada's Pacific coastal province of British Columbia late on Saturday.

The US Geological Survey said the quake was centred 123 miles (198km) south-southwest of Prince Rupert at a depth of 6.2 miles (10km).

The Earthquakes Canada agency said the quake in the Haida Gwaii region was followed by numerous aftershocks as large as 4.6 and that a small tsunami has been recorded by a deep-ocean pressure sensor.

In Hawaii, tsunami warning sirens could be heard out across Honolulu, the state capital on Oahu, the state's most populous island, prompting traffic jams.

At cinemas, films were halted in mid-screening as announcements were made urging customers to return to their homes. The last time Oahu had a tsunami warning was after the devastating Japanese earthquake of March 2011.

On Honolulu's famed Waikiki Beach, residents of high-rise buildings were told to move to the third floor or higher for safety.

Stephany Sofos, a resident of Diamond Head near Waikiki, said most people had either evacuated or relocated to a higher floor.

"I moved my car up the hill, packed up my computer and have my animals all packed and with me," Sofos said, saying that she had not yet seen any obvious receding of the surf, a telltale sign that a tsunami wave is imminent.

"I'm pretty confident because we have a lot of reefs out there and that will prevent any major damage. Maybe it's a false confidence, but I'm not really worried," she said, adding, "It is nerve-wracking."

Tsunami Warning Centre geophysicist Gerard Fryer said the tsunami had caught scientists by surprise.

"We thought that the earthquake was on land and when we learned that it was deeper undersea and we gathered more information, we had no choice but to issue a warning," he said

As residents scrambled to reach higher ground on Oahu, at least four major road accidents were reported by the state emergency medical services. More accidents were also reported on the outer islands.