Mount Pisgah will remain closed to the public through at least the weekend after a large wildfire broke out that forced the evacuation of more than 20 people at the popular hiking spot Thursday.

The Oregon Department of Forestry reported the fire, which started about noon, had grown to about 50 acres late Thursday afternoon. No injuries were reported.

The county said the Howard Buford Recreation Area, Pisgah's home, will remain barricaded until the closure is lifted.

County officials urged residents to respect the closure for their own safety and also so that responders can focus on fire-fighting efforts, tweeting that "looky-loos" were making it difficult for responders to access the area.

Three helicopters, three aircraft and about 85 firefighters and other personnel remained on scene. The cause of the fire wasn't immediately known.

"We are aggressively attacking this fire and plan to have ground crews work throughout the night and as long as needed to contain the fire completely," said Michael Currin, a forester with the Oregon Department of Forestry, which is managing the response.

"To maintain quick access to the fire and safety for all concerned, we ask that the public please avoid the area until further notice."

Dave Kjosness, spokesman for the Oregon Department of Forestry, left open the possibility that the closure of the recreation area could extend into the new work week.

"We want to sure that this (fire) is nice and cold and approachable for the public before we open it back up," he said.

The fire wasn't threatening any homes or structures. The only home at Mount Pisgah is a caretaker's dwelling near the arboretum on the other side of the park.

Lane County Parks Director Brett Henry said more than 20 people were evacuated from the recreation area after the fire started. One elderly couple was slowly making their way down the mountain but they were out of any danger.

The fire, which sent a column of smoke in the air visible from the metropolitan area, appeared to be the biggest reported at Mount Pisgah since a blaze that started near the West Summit Trail blackened 117 acres in August 1999.

Pleasant Hill resident Al Clark, who watched the fire from a road near the recreation area after hearing the sirens, said winds help push the flames up the mountain's slope rather than toward the recreation area's boundaries.

"Initially, it wasn't in the trees at all," he said. "It was just in the grass valley."

Jason Blazar, stewardship director at Friends of Buford Park and Mount Pisgah, credited volunteer clearing of overgrowth and downed trees in the area for helping slow the spread of the fire.

"It hasn't been racing or jumping from spot to spot," said Blazar, while directing traffic at the recreation area's east trailhead.

The mountain rises 1,531 feet above sea level and is the dominant geological feature of the 2,363-acre recreation area. The area has about 17 miles of hiking trails.

The county, which sent a wireless emergency alert to people in the area telling them of the evacuation, reported the fire was burning between trails 2 and 6 and west of Trail 46.

In addition to the state forestry and county parks departments, Lane County Emergency Management, Oregon State Police, Oregon Department of Transportation, local rural fire departments and multiple private contractors responded to the fire. The Willamette National Forest was sending hand crews.

Follow Christian Hill on Twitter @RGchill. Email chill@registerguard.com. Follow Dylan Darling on Twitter @DylanJDarling. Email dd@registerguard.com.