A shot has been fired at police by republicans in north Belfast and there have been other disturbances after Orange Order parades.
At least two police officers have been injured in the city and water cannon used and baton rounds fired to disperse rioters.
Sinn Fein's Gerry Kelly blamed the Real IRA for the trouble in north Belfast.
Elsewhere, three police officers were injured after they were attacked at a parade at Rasharkin, County Antrim.
The officers sustained minor injuries when they were struck by stones and bricks by youths in the village.
Petrol bombs were also thrown. One man has been arrested.
The majority of parades in Northern Ireland passed off peacefully.
Earlier, a 30-year-old woman died after being hit by a car leaving an Orange parade in Killyleagh.
A nine-year-old boy sustained a leg injury in the accident and has been taken to hospital.
The woman was airlifted to hospital by the police helicopter at about 1630 BST to Musgrave Park Hospital and then transferred to the Royal Hospital.
Members, bands and supporters have participated in 18 demonstrations. The annual parades mark the Battle of the Boyne in 1690.
ORANGE ORDER RESOLUTIONS
Faith: Declares it is a Christian organisation which embraces cultural, social and political aspects
Loyalty:Reaffirms its devotion to the Queen and urges members to set an example of good citizenship
The State: Says it stands firm for the Union and reaffirms its commitment to the Act of Settlement of 1701
The Orange Order's three 'flagship' parades this year were in Banbridge, Bangor and Larne.
For the first time, most of the shops in Belfast city centre were open.
Belfast city centre manager Andrew Irvine said a programme of on-street entertainment with street performers "from as far away as Australia and America" would entertain shoppers from 1230 BST.
Stores closed again at about 1630 BST when Orangemen and women were making their return journey.
Speaking at the Twelfth demonstration in Bangor, County Down, the Grand Secretary of the Orange Order, Drew Nelson, said nationalists and republicans needed to reach out to Orangemen and women.
He claimed the Order had shown "goodwill and respect to the nationalist and republican community" but that goodwill had not been returned.
Mr Nelson said his organisation had been the target of unprovoked attacks and had been compared to the Ku Klux Klan, a reference which he found offensive.
He said he was laying down a challenge to the political leadership of nationalism and republicanism to show "a greater understanding and tolerance of the Orange institution".
The main parade passes through Belfast City Centre
Meanwhile, the grandmaster of the Grand Orange Lodge of Ireland, Robert Saulters, has said Protestants in Northern Ireland are "into a situation of ethnic cleansing".
His comments follow a number of attacks on Orange halls recently.
"The thing is we are in a peace process, I would say we are more into a situation of ethnic cleansing, we had seen that in operation on the Ormeau Road in Belfast in the 1970's and it did work for Sinn Fein /IRA, but don't tell the government that, they may not be able to sleep in their beds at night," he said.
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