In December 1955, Rosa Parks was arrested for not giving up her seat for a white passenger
The NAACP decided to hold a bus boycott on the day of her arrest. In the days before social media, it was difficult to get information to the entire black population of Montgomery in a day or two, so they got the preachers to spread it.
Martin Luther King Jr. was looking out his window the morning of the trial, and was shocked to see so many empty buses.
People began walking to work, or carpooling. Some of them would get brought to work by their employers.
The police discovered an old, vague law that prohibited boycotts, and Martin Luther King and the other leaders were arrested.
Eventually segregation ended and the boycott stopped.