The station agent called to report that a male suspect, who is known to her, had exposed his penis to her and began masturbating as he descended the escalator to the platform. The male boarded a train, which was intercepted by officers at the Powell Street Station. The male, who is also known to officers, was off boarded and detained. The agent responded to the Powell Station and identified the detainee as being the one she saw masturbating. The suspect was placed under citizen's arrest and booked into the San Francisco County Jail for Indecent Exposure.