BOULDER, Colo. – Wind-driven wildfires swept across Boulder County grasslands Wednesday, destroying at least four structures and prompting mandatory evacuations of at least 500 homes.

Residents were ordered to leave more than 500 homes on the north side of Boulder, county spokeswoman Barbara Halpin said. At least one other neighborhood north of the city was evacuated, but the number of homes wasn't immediately known.

No injuries were immediately reported.

The Red Cross said at least 100 people went to one emergency shelter. Authorities said more than 11,000 homes had been alerted to the fire by reverse-911 calls, but not all of those households were told to leave.

The fires started in parched, rolling grasslands dotted with subdivisions, individual homes and horse ranches about 25 miles northwest of Denver.

Authorities said at least three of the destroyed structures were homes, and the fourth was either a barn or a home.

The largest fire, which burned more than 10 square miles, jumped across U.S. 36 and entered the Lake Valley Estates neighborhood, where police went from house to house warning residents to evacuate.

There were no immediate reports of houses burning there.

Flames licked the shoulders of one road as cars and pickups rolled by, some towing livestock trailers.

"It's really terrifying," said Pamela Taylor of nearby Louisville, who was trying to find out whether her horse, Zorro, had been moved from a boarding stable in an evacuation area.

An emergency animal shelter at the Boulder County Fairgrounds in nearby Longmont was full, and an overflow shelter was set up at a ranch.

Sheriff's Cmdr. Phil West said the fire apparently started when winds knocked down a power line.

"This is what scares everybody to death, these high winds," said John Stobbelaar, a retired captain with the Mountain View Fire District who went to one of the fires after he was called up on standby. Two smaller fires were contained by early Wednesday evening.

Winds at the scene ranged from 35 to 55 mph, said Kyle Fredin, a National Weather Service spokesman.