A transgender woman was arrested at the Florida State Capitol last month after she used a bathroom there to protest a state law that blocks transgender people from using a rest room that aligns with their gender identity.

The transgender woman, Marcy Rheintgen, 20, said on Sunday that she had intentionally broken the law. Civil rights experts said that this was the first known case of someone being arrested for challenging a law that bans transgender people from using bathrooms in government buildings that do not align with their gender at birth.

Ms. Rheintgen said she had been feeling emotional about what she described as dehumanizing language used by elected officials to talk about transgender people when the idea came to her: “What if I just broke the law because the law is so stupid?”

Ms. Rheintgen said that she sent 160 letters to state representatives, the attorney general and governor to tell them when she would use the bathroom and asked that she not be arrested.

She arrived at the bathroom on the second floor of the House office building of the Capitol in Tallahassee on March 19.

Two police officers spoke to Ms. Rheintgen outside the bathroom and told her that she would be given a trespass warning if she entered, according to an arrest report from the Florida Department of Law Enforcement.