Northern Irish police were dealing with “significant disorder” on the streets of Belfast after trouble flared following a Protestant march through the city.
Police vehicles have been damaged and missiles hurled at officers trying to restore order, a police spokeswoman said.
The rioting broke out in east Belfast, a predominantly Protestant sector of the Northern Irish capital, following the “mini Twelfth” parade.
Police used rubber bullets and a water cannon to try and disperse the crowds, the spokeswoman said.
“Police are currently dealing with significant disorder,” she said.
Officers were working with community leaders and advising members of the public “to avoid the area as they work to restore calm”.
Northern Ireland saw some of its worst sectarian violence in years last week, focused on a Catholic enclave in east Belfast.
A photographer was shot in the leg and rioters threw petrol bombs and other missiles at police, who responded by firing water cannons.
The 1998 peace accords largely ended the cycle of sectarian bombings and shootings in the province, part of the United Kingdom, and paved the way for a devolved, power-sharing Northern Irish Assembly.
However, as before, there are often clashes around the July Protestant marching season, when the light summer evenings are at their longest.