At least seven police officers have been injured after coming under attack by crowds of rioters in west Belfast.
Petrol bombs and missiles, including stones and bricks, were thrown at officers as trouble flared throughout the night, the PSNI said.
A bus was also hijacked and driven at a police cordon, but crashed a short distance away, according to the force, while gun shots were also heard.
Riots broke out in the nationalist areas of Broadway, Old Park and North Queen Street in the west of the city late last night and have continued throughout the early hours of this morning.
The trouble came as loyalists prepared for the traditional Twelfth of July celebrations which mark the 1690 Battle of the Boyne.
Tens of thousands of Orangemen and bandsmen are due to take to the streets later today as part of the annual event, which signals the height of the marching season.
Police used water cannons and fired baton rounds to try and disperse crowds of around 100 and 200 people who began throwing stones and missiles at police lines in the Broadway area of the city.
A number of vehicles were reportedly hijacked with a motorbike and at least one van set on fire, according to police.
Petrol was also thrown at officers in North Queen Street where around 40 people have gathered, the force said.
A bus was hijacked on the Falls Road and then driven at a police cordon separating loyalists and nationalists on the Donegall Road, but crashed before reaching officers.
The PSNI said it was investigating reports of a number of gun shots in the Broadway and Village areas, but had no reports of any related injuries.
A number of injured police officers were taken to hospital, but none of their injuries are said to be life threatening, the PSNI said.
Before the riots broke out, politicians and churchmen on both sides had appealed for a day free of violence, especially in the aftermath of serious disturbances in loyalist areas of east Antrim at the weekend.
Security chiefs have put extra resources on standby in potential flashpoint areas of Belfast and Craigavon, Co Armagh, while every available police officer will be on duty at today's 19 separate demonstrations.
New armour-plated police Land Rovers - part of a replacement consignment of 60 which has just arrived - will give officers additional protection in areas like Ardoyne, north Belfast, where republicans opposed to the peace process have threatened protests.
There has been serious violence in this area before because of local opposition to Orange parades.
The largest parade will be in Belfast where some city centre department stores are planning to open.
The Orange Order leadership insists they are a unique opportunity to showcase its history and heritage and draw many overseas visitors.
Grand master Edward Stevenson said: "There is no other single event that can produce crowds like the Twelfth. It is such a special day of religion, culture, music and pageantry."
A firefighter was injured and a fire appliance damaged during the serious street violence overnight.
Crews from Springfield fire station in west Belfast came under attack and a vehicle windscreen was smashed by stone-throwing youths. A firefighter was slightly hurt by young people throwing stones in Londonderry.
The Northern Ireland Fire and Rescue Service said it had received 180 calls up to 1am this morning, a 65% increase from last year. At the busiest time, the service was taking a call every 75 seconds, with most within the Greater Belfast area.
Deputy chief fire officer Chris Kerr said the NIFRS would continue to engage with community representatives to ensure public safety.
"Firefighters are already exposed to significant risk, in what can be a dangerous profession, without having to face such deliberate attacks from those who they are trying to serve," he said.
© Press Association