Twenty-two police officers have been injured during rioting in Northern Ireland.
Baton rounds were fired and water cannon used in parts of west Belfast after petrol bombs, stones and bricks were launched at police lines.
Four wounded officers were taken to hospital, but none of their injuries were life-threatening, the Police Service of Northern Ireland (PSNI) said.
A bus was hijacked and driven at a police cordon, but crashed a short distance away, according to the force.
A fire service appliance was attacked by missile-throwing youths while attending a blaze in west Belfast and ambulance crews were also targeted.
The trouble happened on the night before today's Twelfth of July celebrations by Orangemen, during which tens of thousands of members of the loyal orders will walk the streets to commemorate the 1690 Battle of the Boyne victory of a Protestant over a Catholic monarch.
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 continued through the early hours of this morning. More than 40 petrol bombs were thrown.
Police used water cannon and fired baton rounds to try to disperse crowds of between 100 and 200 people who began throwing stones and missiles at police lines in the Broadway area of the city. A total of 51 rounds were used.
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 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.
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 (NIFRS) said it received 180 calls up to 1am, a 65% increase on 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.
PSNI assistant chief constable Dave Jones praised colleagues for their professionalism in containing the violence.
"We would appeal for everyone to do everything that they can to help ensure all areas are peaceful over the next 48 hours. Violence does not need to be inevitable," he 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 such as 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 the parades are a unique opportunity to showcase their history and heritage and they draw many overseas visitors.
An Orange Order feeder parade passed the Ardoyne shopfronts in North Belfast, where there has been trouble in previous years, without incident.
Nationalist residents held a silent protest.
Police in Portadown attended reports of public disorder offences in Craigwell Avenue and Obin Street in the early hours of this morning.
Houses were damaged as stones were thrown in the area and at police. One arrest was made for riotous behaviour and criminal damage. Police said two officers were injured during the disturbances.
At around 5.05am, several bottles were thrown at police in the area, with one person reported for disorderly behaviour.
Sinn Fein Assembly member Jennifer McCann said: "Last night's riot was caused by anti-social element coming together at one spot in West Belfast, attacking the police and destroying the local area.
"There was clearly no rationale for gathering last night at Broadway other than the purpose of having a riot.
"Those behind it are not welcome here and they are the very same people who are responsible for anti-social behaviour throughout the year within West Belfast.
"They have left this community in shock. Children were terrified in their homes, people were fearful that their cars might be hijacked, people were fearful that their homes might be attacked."
Nationalist SDLP Belfast City Council member Tim Attwood said the community had been left reeling.
"The Broadway area of West Belfast has been left on its head, having been encroached by violent youths who are intent on inciting fear in this community, causing harm to our emergency services and destroying property," he said.
"Those responsible are doing the people of West Belfast a grave disservice and their futile actions have left residents feeling utterly disgusted."
A loyal order parade today peacefully passed Short Strand in East Belfast where violence erupted last month.