Dhaka, April 4 -- A transgender college student, Marcy Rheintgen, was arrested last month for entering a women's restroom at the Florida State Capitol-an act that civil rights attorneys say marks the first known arrest under transgender bathroom laws passed in multiple U.S. states.

Before being detained, 20-year-old Rheintgen declared, "I am here to break the law," as she entered the building in Tallahassee on March 19. Capitol police, who had been alerted in advance, warned her she would face a trespass notice if she proceeded into the restroom. According to an arrest affidavit, Rheintgen entered to wash her hands and pray the rosary, and was arrested after refusing to leave.

She has been charged with misdemeanor trespassing, an offens...