Some 47 police officers were injured in sectarian rioting in Belfast, after violence flared around a republican parade, police said Monday.
Loyalists from the Protestant community attempted to disrupt Sunday's march by Catholic republicans, with both sides then attacking the police keeping them apart.
Officers were pelted with masonry, petrol bombs and fireworks. Four were taken to hospital, while 43 others were treated for various minor injuries.
"I am both angry and sad that my officers have been subjected to such significant attack," said Northern Irish police chief superintendent George Clarke.
"Every hour of disorder in Northern Ireland not only puts lives at risk, it also reduces confidence in our community and wastes huge amounts of money that could be better spent on schools and hospitals.
"Violence cannot be a solution."
The situation had calmed down by around 2:00 am (0100 GMT) Monday.
Around 3,500 people died in the three decades of sectarian violence between Northern Ireland's Catholics and Protestants that largely ended with a 1998 peace agreement, though sporadic unrest and bomb threats continue.