In New York at the time, it was illegal to kiss or hold hands with other people of the same gender. Police would regularly go undercover to entrap gay men and raid bars that knowingly served liqiuor to three or more homosexuals.
Eight officers entered the Stonewall Inn(bar) and began making arrests. The riot started with a Transvestite threw a bear bottle and a lesbian made attempts to escape. Meeting with resistance, the police retreated back into the bar and sealed themselves in. At one point the police opened the door and dragged a heterosexual who was passing by into the bar and beat him severely. Eventually the crowd grew to 2000 and the riots stretched over about a week.