Rampant wildfires driven by 110mph winds have injured at least six people, forced the evacuation of tens of thousands of people and destroyed as many as 600 homes.
People rushed to leave two towns east of the Rockies due to the prairie grass fires near Denver in the United States.
It is believed they were caused by sparks from fallen power lines brought down by the gales on Colorado's drought-parched Front Range, said Boulder County Sheriff Joe Pelle.
Evacuation orders were given to residents in the town of Superior, Colorado, with a population of about 13,000, and then Louisville, home to more than 18,000 residents, the Boulder County emergency management office said on Twitter.
Separately, the National Weather Service office in Boulder tweeted: "All Superior under an Evacuation order. Leave now!"
Within hours, the blaze had swept an estimated 1,600 acres and destroyed as many as 600 homes, Pelle told a news briefing.
He said an entire subdivision of 370 homes went up in flames west of Superior, and that 210 dwellings were lost in the Old Town area of Superior, along with additional residences in the area.
Governor Jared Polis said flames were consuming football fields of landscape in a matter of seconds, calling the conflagration "a force of nature".
Sheriff Pelle continued: "This is the kind of fire you can't fight at all."
He added that fire and emergency personnel were "essentially running ahead of this just trying to get people out of the way. That's all you can do."
Pelle said tens of thousands of area residents were under evacuation orders.
Among the evacuees were a number of patients from Avista Adventist Hospital in Louisville who were deemed especially vulnerable to smoke-inhalation, said Kevin Massey, spokesperson for healthcare network Centura Health.
"We are transferring some ICU and neonatal patients as it's safe to do so," he added.
Six people were treated for injuries from the wildfires at the UCHealth hospital in the neighbouring city of Broomfield, said spokesperson Kelli Christensen, but she didn't specify the nature of the injuries or the patients' conditions.
The extent of property losses was not immediately known, but KMGH-TV showed footage of a home engulfed in flames near Louisville.
A towering plume of smoke from the wildfire was visible in Denver, about 20 miles to the south.
Wind gusts of up to 110 miles per hour were reported in Boulder, according to the National Weather Service, which said fast-moving fires were creating a "life-threatening situation" in the Superior and Louisville areas.
Polis declared a state of emergency allowing use of disaster funding to support emergency response efforts in Boulder County and to allow mobilisation of the Colorado National Guard and other state resources as needed.
The fire on the outskirts of the Denver metropolitan area, left bone dry from an extreme drought gripping eastern Colorado, follows several days of heavy snow in the Rocky Mountains to the west.
