One person was reported missing.
The High Park fire, raging about 15 miles west of Fort Collins, Colo., had grown to almost 60 square miles.
Larimer County Sheriff Justin Smith said the fire was pushed by dry wind and fueled by swaths of trees damaged by beetles.
"This is simultaneously burning through every area we've been individually concerned about for the past two decades," he told KUSA-TV, Denver. "The best thing we have now is prayer for a change in weather."
About 400 firefighters were battling the fire.
CNN said smoke hung over Fort Collins but the city of 143,000 residents was not immediately threatened by the fire.
Crews searched for an unidentified person whose home was destroyed by the uncontained fire.
The person "might not have made it," Poudre Fire Authority Chief Tom DeMint told The [Fort Collins] Coloradoan.
By early Monday, more than 2,200 evacuation calls had been made ordering people from homes in the Poudre, Rist, Redstone and Mill canyons surrounding the fast-growing blaze.
Most evacuations Sunday went smoothly, but some residents refused to leave, DeMint said
"Evacuation is paramount," the newspaper quoted him as saying. "Follow those orders."
The fire is one of three big wildfires blackening more than 70,000 acres in three states. The other two are New Mexico and Wyoming.
The Little Bear fire at Lincoln National Forest near Ruidoso, N.M., was nearly as large as the Colorado fire, covering more than 34,500 acres by Monday afternoon, officials said. It remained uncontained.
Ruidoso was hit by a devastating flash flood from a hurricane four years ago, killing one person and causing $20 million in damage.
Progress was being made in Wyoming by firefighters battling a wildfire that started Saturday afternoon inside Guernsey State Park. The fire had burned more than 2,500 acres inside the park.
State Sen. Jim Anderson, acting as governor of Wyoming while Gov. Matt Mead is in China and Secretary of State Max Maxfield is outside the state, activated the state National Guard Sunday to assist firefighters trying to control the blaze.
Three UH-60 Black Hawk helicopters and their support teams arrived Sunday in Guernsey and were able to start dumping water on the fire, the governor's office said in a statement.