Britain's secretary of state for Northern Ireland says residents must choose between peace and violence, following riots that injured 50 policemen in and around Belfast.
Peter Hain said Monday that video footage of the two nights of rioting showed members of the militant Protestant Orange Order attacking police.
Police officials have blamed the group for inciting the violence, sparked after officials rerouted an annual Protestant parade away from a Roman Catholic area.
Protest leaders have rejected the charges.
Witnesses say rioters blocked roads in and around Belfast, threw gasoline bombs at police and set cars on fire. Gunmen also opened fire on police, who shot rubber bullets to break up angry crowds.
At least 16 people were arrested in the violence, some of the worst in the British-ruled province in several years.