DENVER -- Two teens wearing "March 4 Peace" placards said Rocky Mountain National Park officials violated their free-speech rights when they blocked them from entering the park.

Park spokeswoman Kyle Patterson said the teens' actions Sunday were considered a public demonstration, which national parks regulate as a safety precaution for protesters and to make sure they don't detract from other patrons' use of the parks.

The California teens eventually entered the park after removing the placards and writing the slogans on their shirts.

Ashley Casale, 19, and Mike Israel, 18, have been walking from San Francisco to Washington, D.C., to spread their message. They said they were wearing their placards over their T-shirts when they were detained at an entrance to Rocky Mountain National Park.

Casale said a park ranger asked to remove the bibs and told them they would need a demonstration permit. Park rules require permits for "public assemblies, meetings, gatherings, demonstrations, parades and other public expressions of views."

"They said they couldn't let us in because we were making a political statement," Casale said.

Casale, a student at Wesleyan University in Connecticut, asked park officials to explain how the placards were different from bumper stickers or T-shirts that other visitors wear.

Patterson said bumper stickers and T-shirts are private expressions while the placards were more of a public expression.

"They had placards. They had reporters with them. They were promoting their Web site. It became a public demonstration and public expression," Patterson said.

Alan Chen, a University of Denver Sturm College of Law professor and free-speech expert, said park officials overstepped their authority.

"This is an astounding story," Chen said. "There is no question at all, those people have the right to not only wear the placards but walk through the park with them on."

"To say to two persons who happen to be carrying a message as they walk through the park that they're carrying on a demonstration that requires a permit is ridiculous," said Mark Silverstein, legal director for the American Civil Liberties Union of Colorado.