A Fort Collins ordinance that bars women from exposing their breasts in public is sexist, violates the Constitution and is rooted in outdated puritanical values and discrimination, according to a lawsuit filed Tuesday in U.S. District Court in Denver.

Free the Nipple, an association of Colorado residents who advocate for gender equality, the group’s founder Brittiany Hoagland and Samantha Six, a group member, filed the 12-page lawsuit.

Hoagland and Six appeared at a news conference to announce the litigation dressed in T-shirts bearing an image of a pair of breasts stripped across their chests.

“We do have the complaint and we are in the process of reviewing it,” said Carrie Daggett, Fort Collins’ city attorney. She had no further comment.

Last year, Hoagland brought the ordinance to the attention of the Fort Collins City Council. She wanted a change made to the language so that both men and women would be free to show their breasts in the city.

Last October, the Council rejected the idea but approved tweaking the current law to allow breastfeeding and clarify how the ordinance applies to children younger than 10. That change defined the breast as beginning at the top of the nipple.

“I’m extremely excited that the legal ball is finally rolling,” Hoagland said Tuesday.

Section 17-142 of the Fort Collins code of ordinances discriminates against women by “making it a crime to show their breasts and/or nipples in any place where they might be viewed by others while allowing boys and men to show their breasts and/or nipples at any time or place without fear of arrest or prosecution,” the lawsuit said.

The group believes that any law containing language to the effect of “women are prohibited from” is sexist and, therefore, unconstitutional, according to a news release from Denver civil rights lawyer David Lane, whose law firm filed the case.

The modified version “continues to criminalize women who appear at public places with their breasts and nipples exposed,” the lawsuit said.

Thirty states, as well as Denver, Boulder, and New York, have recognized a woman’s right to appear topless in public, the suit said.

A federal judge in Chicago recently ruled against a woman’s challenge to that city’s prohibition against women showing their breasts.

In January, a federal judge in Springfield, Mo., filed a preliminary injunction stopping that city from enforcing a provision of the indecent exposure law that made it illegal for women to show any portion of the breasts beneath the areola.

Each violation of the Fort Collins ordinance can be punished by a fine of up to $2,650, imprisonment of up to 180 days or both, the suit said.

The lawsuit asks for an injunction to stop enforcement of the law, and declare it unconstitutional.

Indecency codes are largely local, with very few states having laws on the books banning the exposure of female breasts.

Gender-specific indecency laws have received increased attention in recent years. A 2014 film “Free The Nipple” explored the topic, and celebrities including Rihanna and Miley Cyrus, have clashed with social media sites for censoring photos showing their nipples, while not removing photos of topless men.

The Associated Press contributed to this report.