Police and residents said a wave about 10 feet high struck the western town of Gizo, inundating buildings and causing widespread destruction. A man who answered the telephone at the Gizo police station said there were initial reports that eight people, six of them children, had been killed by the tsunami but they were still unconfirmed. The phone cut out abruptly before the man gave his name.
Gizo resident Judith Kennedy said water "right up to your head" swept through the town.
"All the houses near the sea were flattened," she told The Associated Press by telephone. "The downtown area is a very big mess from the tsunami and the earthquake," she added. "A lot of houses have collapsed. The whole town is still shaking" from aftershocks.
The U.S. Geological Survey said the quake measured magnitude 8.0 and struck at 7:39 a.m. about 6 miles beneath the sea floor, 217 miles northwest of the capital, Honiara.
The Pacific region from Australia to Hawaii went on high alert for several hours after the quake struck between the islands of Bougainville and New Georgia, though officials canceled a region-wide tsunami warning after the danger period passed.
Gizo, a regional center, is just 25 miles from the earthquake's epicenter.
Another witness in the town, dive shop owner Danny Kennedy, estimated the height of the wave at 10 feet.
"I'm driving down the street - there are boats in the middle of the road, buildings have completely collapsed and fallen down," he said in a telephone interview.
"We're just trying to mobilize water and food, and shelter for people at the moment because ... in the town alone there's going to be between 2,000-3,000 homeless. It's not a very good scene at the moment."
Harry Wickham, who owns a waterfront hotel in Gizo, said the damage was widespread.
"The waves came up probably about 10 feet and swept through town," he told Australia's Nine Network television by telephone. "There's a lot of water damage and a lot of debris floating around," he added.
"Ten feet of water washing through town - you can imagine what damage it has done here."
Julian McLeod of the Solomon Islands National Disaster Management Office said there were unconfirmed reports that two villages in the country's far west were flooded.
"Two villages were reported to have been completely inundated," McLeod told Australian Broadcasting Corp. radio. "We have received reports of four people missing."
A town in the west, Munda, was believed to be badly damaged, officials and the Solomon Islands Broadcasting Corp. said, but communications were difficult and details were not confirmed.
The Hawaii-based Pacific Tsunami Warning Center reported the quake at magnitude 8.1, and said a temblor of that strength could cause a destructive tsunami and issued a warning bulletin for the Solomon Islands and neighboring Papua New Guinea.
It ordered a lower-level "tsunami watch" for other places, including most South Pacific countries, but later canceled the alert. The center said a 6-inch wave had been reported in Honiara.
Police Sgt. Godfrey Abiah said in Honiara that police in Gizo had received warning about a possible tsunami and were helping people leave the town for higher ground when the wave hit.