Twelfth of July parades are taking place in 17 locations across Northern Ireland.
Tens of thousands are expected to attend the events marking the anniversary of the Battle of the Boyne.
William III – the Dutch-born Protestant better known as William of Orange or King Billy – defeated the Catholic King James II in County Meath in July 1690.
A feeder parade past north Belfast’s flashpoint Ardoyne shops area passed without incident on Thursday morning.
Edward Stevenson, Grand Master of the Grand Orange Lodge of Ireland, said Thursday would “showcase culture and heritage”.
“We are confident of welcoming increasing numbers of visitors, tourists and families to all of our parades,” Mr Stevenson told BBC News NI.
“There are very few events on these islands that can bring such vast numbers of people onto the streets, either by taking part or simply to watch the music and pageantry.”
The Orange Order was formed near Loughgall in County Armagh in 1795, when its founding members pledged their loyalty to the royal family and swore to defend the Protestant faith.
Northern Ireland Secretary Karen Bradley and former Ireland rugby captain Brian O’Driscoll were among those who attended the parades.
Mr O’Driscoll tweeted to say he was at the procession in Loughall filming a documentary on how rugby unifies Ireland.
On 12 July, marching bands from Orange lodges all over Northern Ireland parade through villages, towns and cities before listening to speeches and prayers by senior Orangemen.
On Saturday, thousands of Orangemen from the Republic of Ireland and Northern Ireland took part in the annual Orange Order parade in Rossnowlagh.
About 50 lodges from Cavan, Leitrim and Monaghan – as well as the host County of Donegal – marched along the narrow country roads into the seaside village.
This year, parades are being held in: