Some residents are being allowed back to their home to retrieve valuables and pets but are being warned not to stay.
The cause of the fire is under investigation.
Firefighters have controlled a second, smaller fire nearby.
Authorities abruptly closed off access to one neighborhood that was in the fire's path but had not yet been evacuated, blocking residents trying to get in to check on their property or retrieve pets and possessions.
David Goldberg was standing at a roadblock waiting for his wife to drive down from their house with their four dogs.
"I told her, 'Just leave,'" said Goldberg, an attorney. "We've been through this before."
Goldberg said the family was still planning to catch a 5 a.m. Saturday flight for a vacation in Hawaii.
"It's just part of life," he said of the fire, adding that material possessions were not overly important. "Things are just things."
Mark Martina, a mortgage broker, was heading home to get his dog when he reached the roadblock not far from his house.
"It's unbelievable," he said. "It's pretty ridiculous to shut things down and not let anyone know."
Angela Dietrich, whose home was not in the fire's immediate path, said the smoke was so bad she couldn't see her yard from inside the house.
She said she was amazed the wildfire sprang up so early in the year.
"This is a really bad start," she said.
Poudre Fire Capt. Patrick Love said aircraft have been requested to fight the fire but he doesn't believe any are available.
"I don't think they're in the area. Not this time of the year," he said.
The fire came as much of the state dealt with drought conditions after a relatively dry winter. The snowpack in the mountains was low, leaving farmers wondering how many crops to plant and raising the possibility of lawn-watering restrictions along the Front Range.
Temperatures reached into the 70s along Colorado's Front Range, record-breaking territory for March. Part of Horsetooth Reservoir, however, is still frozen over.
Colorado's wildfire season also started in March last year.