Police have fired a number of baton rounds after being attacked with petrol bombs by rioters in north Belfast.
More than 100 riot police were involved in an operation to remove nationalist protesters blocking the road ahead of an Orange Order march in Ardoyne.
A crowd threw petrol bombs, a blast bomb and other missiles at police.
A policewoman suffered head injuries during the violence, which Assistant Chief Constable Alistair Finlay described as "totally unacceptable".
In a statement, he said the past 24 hours had been "a very challenging time for policing".
The policewoman was struck on the head by debris thrown from the roof of the shops at about 2200 BST.
Police said they had to protect their injured colleague and ambulance crews from further missiles in Ardoyne as they tended to her, before she was taken to hospital.
The parade went by the flashpoint area at Ardoyne shops. As it passed, stones and bottles were thrown.
The march was delayed for about 90 minutes because of the protests.
A heavy security presence remains in place in the area.
Sporadic trouble broke out in parts of Northern Ireland as Protestant Orangemen staged their annual parades.
On the eve of the marches, three police officers were shot during rioting.
The Twelfth of July is the annual high-point of the loyal orders' parading calendar.
Some marches have been a source of tension between nationalists who see the parades as triumphalist and intimidating, and Orangemen who believe it is their right to walk on public roads.
On the Ormeau Road bridge in south Belfast, petrol bombs and paint were thrown at police on Monday evening.
A car was later set alight on the bridge.
A police spokesman said they were also dealing with minor disturbances on the Albertbridge Road and Short Strand area.
Elsewhere, police in riot gear responded to what a PSNI spokesperson described as "a major disturbance" at Botanic train station in south Belfast.
Dozens of police officers moved into the area after a disturbance on the platform.
Two men and a woman are to be reported to the Public Prosecution Service for alleged disorderly behaviour and assault on the police.
In west Belfast on Monday afternoon, a bus driver was forced to drive to a police station by two masked men who claimed they had left a bomb on the upper deck.
It is understood the men boarded the bus at Glencolin Walk shortly before 1600 BST.
He drove the bus to Woodbourne police station. Police later said the alert was a hoax.
In the north of the city, a number of cars were hijacked in the Oldpark Road area.
In Lurgan, County Armagh, youths halted a train in the Lake Street area at about 1630 BST and attempted to set it on fire, but the driver managed to restart the vehicle.
None of the 55 passengers on board the Belfast to Dublin are believed to have been injured.
A van was also hijacked in the area.
Later, police said they were dealing with sporadic violence in the town's Antrim Road area, with petrol bombs being thrown at officers.
In Armagh city, police were investigating reports that a vehicle was set on fire on the Killylea Road and that a large number of youths had gathered in the area.