LYONS, Colo. — With rain still falling and the flood threat still real, authorities called on thousands more people in the inundated city of Boulder and nearby towns to evacuate as rivers and creeks rose to dangerous levels.

The late-night reports from Boulder and the village of Eldorado Springs came as rescuers struggled to reach dozens of people cut off by flooding in Colorado mountain communities. Residents in the Denver area and other downstream communities were warned to stay off flooded streets.

The towns of Lyons, Jamestown and others in the Rocky Mountain foothills have been isolated by flooding and without power or telephone since rain hanging over the region all week intensified late Wednesday and early Thursday.

At least three people were killed and another was missing, and numerous people were forced to seek shelter up and down Colorado’s populated Front Range.

Boulder County spokesman James Burrus said 17 people were unaccounted for Friday, and officials planned to publicly release the names later.

“Unaccounted for doesn’t mean missing. It means we haven’t heard back from them,” he said.

Two backpackers who were hiking the 14,259-foot Longs Peak became stranded when the weather turned, the mother of one of the backpackers said today.

The University of Colorado in Boulder canceled classes at least through today after a quarter of its buildings were flooded.

Interstate 25 east of Loveland was closed in both directions today, state transportation officials said.

In Fort Collins, neighborhoods along the Cache La Poudre River were evacuated overnight, with the river expected to rise to nearly 2 feet above flood stage this morning, according to the National Weather Service.