RIVIERA BEACH, Fla. (AP) _ About 500 homes were damaged or destroyed by a tornado that touched down Thursday in north Palm Beach County, flipping semitrailers, snapping power poles and tearing roofs off businesses. Only minor injuries were reported.

Most of the damage took place north of West Palm Beach. At the Garden Walk mobile home park, the tornado collapsed the roofs and walls of several homes and pushed others off their foundations. At least two tractor-trailers nearby were tossed upside down.

About 21,000 people were without electricity in Palm Beach County. Power was out in most of Riviera Beach, where reports of damage were extensive. The roof of a Pepsi-Cola plant was torn off, heavy rain caused street flooding and closed roads, and looting was reported in the storm's wake.

At Driftwood Plaza in nearby Jupiter, a metal roof covering 32 stores was damaged, said Tammy Goodwin, owner of the Coastal Massage Day Spa.

``We were right in it,'' she said. ``Pieces of the roof were flying off. A pole came down. There was a lot of damage, a lot of debris. Things were flying.''

Red Cross spokeswoman Laura Botting said two shelters had been opened in Palm Beach County as of 9 p.m., and that other shelters may open if needed.

The weather service said the tornado was spotted at 5:13 p.m. and cleared out by 5:40 p.m. Forecasters were surprised by the rapid development of the system.

``It caught us,'' forecaster Bob Ebaugh said.