HONOLULU, Hawaii — A potentially destructive Pacific tsunami hit Hawaii Sunday after a powerful earthquake struck off the west coast of Canada, triggering calls for urgent action to protect lives as sirens wailed across the archipelago.

"The tsunami is arriving right now," Gerard Fryer, a senior geophysicist with the center, told reporters. "It is coming in as we speak."

Countless Halloween parties were interrupted, restaurants and bars emptied, and highways quickly filled with cars heading away from beach areas.

Television images from the island of Oahu showed relatively small waves peacefully rolling toward shore.

But Fryer urged Hawaii residents not to deceived by appearances.

"Typically, the first wave is not the largest," he said, adding that subsequent waves could be much larger, resulting in flooding in low-lying areas.

"If the waves are big, the all-clear may take six or seven hours," the scientist said.

Initially, the Pacific Tsunami Warning Center said there was no "destructive widespread tsunami threat" after the 7.7 magnitude quake shook the Queen Charlotte Islands off the west coast of Canada.

But later it issued a warning, saying a tsunami had been generated by the earthquake and that it was headed toward Hawaii.

Sirens blared across the archipelago as local officials took to the airwaves, urging residents to head for higher ground -- or higher floors if the were in multi-story buildings.

But local officials were adamant that timely warnings were about all they would be able to do, and execution of evacuation plans depended on individuals themselves.

"We have done everything we can to get the information out," said Honolulu Mayor Peter Carlisle. "Everybody is getting out. You can't rely on the police because they are being pulled out, too."

The epicenter of the Canadian quake, which occurred at 8:04 pm Saturday (0304 GMT Sunday) was located 139 kilometers (86 miles) south of the town of Masset, the US Geological Survey said.

Numerous aftershocks, some as strong as magnitude 4.6, followed the initial quake, Canadian officials reported.

Emergency officials in British Columbia urged residents in low-lying coastal areas to be alert to instructions from local officials and be prepared to move to higher ground.

"The tsunami alarm went off and everybody went to the evacuation site," Danny Escott, owner of the Escott Sportfishing lodge near Masset, told AFP by telephone.

But officials in Canada sought to calm the population.

"We would not be expecting any widespread damage or inundation," Kelli Kryzanowski of Emergency Management British Columbia told reporters during a teleconference.

Natural Resources Canada said in a statement that the quake was felt across much of north-central British Columbia, including Haida Gwaii as the Queen Charlotte Islands are also called, Prince Rupert, Quesnel, and Houston.

But the ministry also played down the effects on Canada, saying: "There have been no reports of damage at this time."

However, Fryer said quakes exceeding magnitude 7.0 should not be taken lightly.

"A 7.7 is a big, hefty earthquake. It's not something you can ignore," said the geophysicist.

He said it had struck partly under an island, but mostly under shallow water.

"I think we have to be thankful it happened where it did," Fryer said. "If that were a heavily populated area, it would have caused significant damage."

The earthquake reading was based on the open-ended Moment Magnitude scale used by US seismologists, which measures the area of the fault that ruptured and the total energy released.

The Queen Charlotte Islands, which are also known by their official indigenous name of Haida Gwaii, comprise about 150 islands located north of Canada's Vancouver Island. Their total population is about 5,000. The Haida people make up about 45 percent of the total population.