630 extra police officers drafted in for Northern Ireland parades
The 30 units of specially trained public order officers will come from forces across England, Scotland and Wales.
A total of 550 parades are due to take place throughout Northern Ireland on Friday.
The Police Service of Northern Ireland Chief Constable said the additional officers, trained in the run-up to last month's G8 summit, would help to police the 43 marches deemed to be contentious.
They will not be posted to potential flashpoints, for example Ardoyne in north Belfast where the threat of disorder is high.
Chief Constable Matt Baggott said: "This is a unique year. If you look at the scale - 550 parades, we have 43 that are sensitive - it would be remiss of me not to plan for every eventuality.
"This particular year I thought it was wise and the right thing to do to bring people over."
The Chief Constable insisted the PSNI could cope and claimed violence was not an inevitability of the marching season.
He added: "I never worry but I plan for every contingency.
"When you have large numbers of people on the streets our primary concern is about safety.
"We don't have, every year the breadth and depth of this neither do we have the volatility we saw in the first three months of this year.
"I am not worried, I am not planning for disorder but as the chief constable we have meticulously planned for every eventuality."
Last year police were pelted with petrol bombs, bricks and other missiles when trouble flared following an Orange Order parade past the Ardoyne interface.
Shots have also been fired and an explosive device was thrown at police lines during clashes with protesters.
Talks between nationalist residents opposed to the annual march and the Orange Order ended without agreement.
The threat from dissident republican terrorists opposed to the peace process remains at a severe level in Northern Ireland according to the Home Office.
The additional mutual aid officers will be equipped with the same kit as their Northern Irish colleagues but will not be deployed without the assistance of an armed PSNI officer.
PSNI Assistant Chief Constable Will Kerr said: "We are concerned that there is a series of very complex dynamics at play in Northern Ireland and in Belfast on July 12 that is going to be exactly the same.
"What we have to do is make sure there are enough resources to manage all those competing dynamics that includes the severe threat from dissident republicans."
Earlier this year violence erupted in parts of Northern Ireland after a decision to lower the flying of the Union flag over Belfast City Hall.
Loyalists staged pickets and blocked roads in a failed attempt to have the decision reversed.
Meanwhile, the five main political parties in the Stormont Executive appealed for calm over the Twelfth. In a joint statement they said: "We appeal to community leaders and, indeed others, such as parents to seek a peaceful parading season to avoid an impact on our citizens, through damaged community relations or the life-restricting consequences of criminal records."