The ancient Roman and Byzantine Empires had well developed associations of sports fans, especially for chariot racing, a popular diversion for the poor. There were four major factions of chariot racing fans, signified by the colour of the uniform of their favourite racing team. These were the Blues, the Reds, the Greens, and the Whites, although by the Byzantine era the only teams with any influence were the Blues and Greens. The teams had aspects of street gangs and political parties, grouping people by social class and religion. The emperors tended to support one faction over the other; Justinian I was, until the riots, a supporter of the Blues.
In 531, some members of the Blues and Greens had been arrested for murder, but had survived their hanging sentences. Justinian commuted their sentences to imprisonment, but the Blues and Greens demanded that they be pardoned entirely. Justinian ignored them, so on January 11 they broke into the prison and set fire to parts of Constantinople, using the racing cheer nika ("victory") as a rallying cry.
Some of the Senators saw this as an opportunity to overthrow Justinian, as they were opposed to his new taxes and lack of support for the nobility in general. The rioters, now armed and probably controlled by their allies in the Senate, also demanded that Justinian dismiss the prefect John the Cappadocian, who was responsible for tax collecting, and the quaestor Tribonian, who was responsible for rewriting the legal code. They then declared a new emperor, Hypatius, who was a nephew of Emperor Anastasius I.
Justinian considered fleeing, but Theodora convinced him to stay in the city. Justinian had his generals Belisarius and Narses suppress the revolt on January 18, which they did with much bloodshed by trapping the rebels in the Hippodrome. About thirty thousand rioters were reportedly killed. Justinian also had Hypatius executed and exiled the senators who had supported the riot.
The Guy Gavriel Kay fantasy novel Sailing to Sarantium depicts a revolt inspired by, and very similar to, the Nika riots.