A total of 18 demonstrations are being staged to mark the 325th anniversary of King William III's victory over James II at the Battle of the Boyne in 1690.
The largest event will be held in Bessbrook, Co Armagh where Orange Lodge of Ireland Grand Master Edward Stevenson will address the crowds.
More than half of the 3,000 police on duty are based in Belfast where a massive march through the city centre is under way.
Police rolled out a massive security operation in Ardoyne, a volatile community flashpoint in the north of the city where a bitter dispute over a contentious parade remains unresolved.
A morning feeder parade passed the area without incident, and hopes are high that the situation will remain calm for the return leg later.
Holy Cross parish priest Fr Gary Donegan said: "It was one of the most peaceful parades we have had.
"The community just want to it all to be over. If the morning parade goes through peacefully and there is no return -- it's the perfect solution for this community."
The Parades Commission - a Government-appointed body set up to rule on controversial marches - gave permission for the morning parade to go ahead with certain restrictions, but has prohibited Orangemen from passing along the road when they return from traditional celebrations elsewhere.
Instead, marchers and bandsmen will be stopped at police lines in the unionist Woodvale area.
In 2013, loyalists clashed with police - but last year significant efforts, including the use of a large number of Orange Order and community marshals, helped ensure there was no repeat of the mass violence.
For the past two years, loyalists have manned a protest camp and staged nightly parades at Woodvale - requiring a policing operation costing millions.
Nationalist residents from the Crumlin Ardoyne Residents Association (Cara) picketed the morning parade with posters urging dialogue.
Sinn Fein MLA Gerry Kelly, who was at the scene, said he was hopeful for a peaceful Twelfth.
He said: "It is has been the quietest bonfire night in north Belfast in a considerable period of time -- maybe 15 or 16 years.
What we had here this morning was a substantial protest - it was very quiet, very calm and that is the way protests should be.
"I think we are in a better place - we are getting to a better place, we just have to have the stamina to keep talking.
"I hope the evening will follow as the day has passed in a calm and quiet way."
Despite calls for "as many people as possible" to turn up, only around 100 supporters joined the protests organised by the hardline Greater Ardoyne Residents Collective (Garc) against the parade.
Garc was widely blamed for orchestrating riots when the parade was given the go-ahead.
There is expected to be a significant police presence in east Belfast where sectarian tensions have increased between communities in the Newtownards Road and Short Strand areas.
A ring of steel is likely to be erected to keep rival factions apart.
The Police Service of Northern Ireland (PSNI) has said it intends to uphold the Parades Commission determinations and will have 58 public order units on standby in case trouble flares.
Contingency plans have also been drawn up to use baton rounds, deploy water cannon and draft in an extra 300 police officers if the situation worsens.
Unlike last year, there has not been a joint call from a broad range of unionist and loyalist political parties, including two with links to paramilitary groups, for the Twelfth to pass off peacefully and lawfully.
Concerns have also been raised about the marshalling of parades - and the PSNI has been lobbying the Orange Order and community leaders on the issues.
One senior security source said: "There is no question things are very tense. There are ongoing interface issues.
"But we have everything to play for. Violence is not inevitable."
The Orange Order has insisted it will marshal parades, but would not step in if violence occurs.
In its pre-Twelfth message, the Grand Orange Lodge of Ireland described a growing sense of resentment at the perceived erosion of loyalist culture - but urged members and supporters to remain peaceful and dignified.
A statement said: "Our struggle will be won by means that have been successful in defeating those who oppose us - perseverance, peaceful protest and prayer."
Ivan Lewis MP, Labour's shadow secretary of state for Northern Ireland, has appealed for politicians to show leadership.
He said: "The marching season is an important time when many people in Northern Ireland celebrate their heritage and identity.
"Northern Ireland's politicians have a crucial role to play in minimising tension, calling for mutual respect and supporting the PSNI in upholding the rule of law.
"I hope they and other community leaders will do all they can to ensure this marching season is peaceful and free of conflict and anti-social behaviour."
