Show Low, Ariz. ? Authorities ordered the 7,700 residents of this eastern Arizona mountain community to evacuate Saturday night as a mammoth fire breached a hastily constructed fire line on its western edge.

The mandatory evacuation orders came as the fire, which had burned more than 150,000 acres of forest since Tuesday, passed a fire line about eight miles west of the city that crews had been building but fled earlier in the day as the blaze approached.

An unidentified Corona de Tucson firefighter flees from a hot spot at a girls camp off U.S. Forest Road 322 in Aripine, Ariz. Crews rushed to reinforce firebreaks Saturday to protect forest communities from two wildfires that threatened to unite.

There was no immediate threat to homes as the fire was moving slowly, fire spokesman Jim Paxon said. Officials expected residents would leave the town over the course of the night.

Meanwhile, a second, smaller fire crossed another fire line and entered Heber-Overgaard, an already evacuated community of 2,700 about 25 miles to the west, Paxon said.

The two blazes, just miles apart, already had forced about 8,000 people to evacuate from several communities before orders came to Show Low. Officials expected the fires to link, creating a 50-mile line.

“Nature’s in control,” Paxon said. “She’s dealing the hand.”

Fire information officer Rob Deyerberg said a merged fire would move faster and burn more fuel, making it harder to fight. “If it merges it just gets worse,” he said.

At nightfall, a dense gray smoke cloud loomed over Show Low. Some drivers pulled over on the side of the road to watch the smoke plume. Others videotaped it.

“It’s a monster. It’s awesome,” said Bobby Smith, who had been staying in Show Low after having already been evacuated from Pinedale. “It’s unbelievable what a big fire can do.”

At least 12 homes and 20 smaller structures were destroyed when the larger of the fires entered Pinedale, 125 miles northeast of Phoenix.

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The biggest of the two fires was thought to be caused by man, although authorities didn’t know whether it was an accident or arson. The second, smaller fire was started by a lost hiker signaling for help.

The area of eastern Arizona, known for tranquil mountains and mild weather, draws hikers and campers and is a summer getaway for Phoenix-area residents escaping the desert heat.

In hard-hit Colorado, crews struggled against an unpredictable blaze in the southwestern corner of the state, but cool weather helped firefighters battling a larger fire south of Denver.

An erratic, wind-driven fire near Durango that has burned 67,700 acres and destroyed 45 homes was only 25 percent contained Saturday.

Residents of 18 subdivisions had been ordered to evacuate their homes, but five of those subdivisions were reopened Saturday, La Plata County Sheriff Duke Schirard said. He did not know how many homes or residents were involved.

At a 137,000-acre blaze south of Denver, firefighters welcomed another day of cool weather.

The fire was 60 percent contained Saturday.