At around 6 p.m. on Thursday 1 December 1955, Rosa Parks boarded a National City Lines bus in Court Square, Montgomery, Alabama. State law mandated racial segregation on all public transportation: on buses, the front rows were reserved exclusively for Whites; black passengers were routinely required to pay their fare at the front of the bus, but could only board at the back, where they frequently had to stand behind empty rows of seats. Parks sat down with three other black passengers in the fifth row, at the front of the seats reserved for blacks; but as the bus began to fill with Whites, the driver told the passengers in Parks's row to move, so that all the white passengers could sit down. Reluctantly, three of the four moved; Parks refused. She later described the ensuing conversation with the bus driver:

Parks was arrested under city and state segregation laws, and bailed to appear in court the following Monday, 5 December.

The racial segregation practised by Montgomery City Lines had for some time been strongly opposed by local black organizations, including the Women's Political Council, whose president, Jo Ann Robinson, taught English at the black-only Alabama State College. Following Parks's arrest, Robinson spent the night at the College, making thousands of copies of a tract calling for a boycott of the buses: the following day, she and her students distributed 35,000 copies of the call across Montgomery. On Sunday, black churches relayed the call, whilst the local newspaper, the Montgomery Advertiser, carried it on the front page.

In court, Parks was found guilty of breaking the segregation laws, handed a suspended prison sentence, and fined $ 10 (plus another $ 4 in court fees). But the boycott was exceptionally successful: almost the entire local black community participated. In the afternoon, the Montgomery Improvement Association was established to coordinate the continuation of the boycott; Martin Luther King, Jr., a young minister recently appointed to the city’s Dexter Avenue Baptist Church, was nominated president.