The National Guard effort to get residents out of Lyons began shortly after daybreak. About 100 troops in 21 heavy vehicles able to ford high waters streamed into the city to begin moving residents out, Gov. John Hickenlooper said.
Residents had been entirely cut off, without water or sewer service, in many cases without electricity, facing what Fire Chief J.J. Hoffman said in a Facebook posting was a "very large disaster."
It was unclear when the evacuation would be complete.
"I encourage all of you -- stay strong!" Hoffman wrote on the fire department's Facebook page. "We will make it through this, we are here for you and doing the absolute best we can with the resources we have to get to each and every one of you!"
As Lyons evacuees arrived at a shelter set up in a church in nearby Longmont, they told stories of houses ripped off their foundations as the St. Vrain Creek turned into a violent river, CNN affiliate KMGH reported.
KMGH reporter Theresa Marchetta said evacuees also described homes dangling off cliffs.
Some people in Lyons still were awaiting rescue, evacuees said, and some residents had chosen to stay. Marchetta said evacuees told her there had been a town meeting and residents were checking on each other to ensure no one was missing.
State transportation officials issued an emergency alert to residents in some of the hardest-hit counties, warning them to stay off roads because many are unstable and could give way without notice. They also closed Interstate 25 from the Wyoming line south to Denver. Part of Interstate 70 also was shut down.
In Fort Collins, some residents had been urged to leave their homes. And in Denver, police responded when a man was swept into a drainage pipe with his dog. Both were saved after traveling two blocks in the water, police said on Twitter.
The rains sent virtually every waterway in Boulder County coursing out of its banks, and massive water flows washed away roads and bridges, flooded homes and stressed numerous other bridges.
In the early hours Friday, flood sirens sounded in Boulder County as emergency officials feared that debris-caked canyons might give way and send another wall of water crashing through the city of Boulder and neighboring communities.
"All residents are warned to go to higher ground immediately due to the potential for flash flooding along the creek," Boulder's Office of Emergency Management said.
Emergency management warned that "there are mudslides at the mouth of Boulder Canyon 400 feet long and four feet deep as the sides of the canyon give way due to the saturation from the days-long rain."
Authorities continued to warn of the danger of mudslides Friday night.
Hickenlooper warned an extensive recovery is ahead.
"This is not going to get fixed in a week," he said. "We have lost a great deal of infrastructure."