The “Orange Order” parades on July 12, which have long been a flashpoint for sectarian tension, included Northern Ireland in its third month without a functioning government after the May election.
In London, the resignation as Conservative leader of Prime Minister Boris Johnson has created more instability, as candidates bidding to replace him are putting up positions in post-Brexit trade policies for the territory.
Across Northern Ireland on Monday, more than 250 bonfires were lit in unionist communities to kick off the festivities.
Fires, often created by stacking flavors on tall structures, have evolved over decades.
Builders in the port town of Larne hope to set a world record with a 200-foot (60-meter) fire.
On Saturday a man in his 30s was killed when he fell from another bonfire more than 50 feet high in Larne. The pyre was subsequently demolished.
The structures and parades of members of the Orange Order commemorate the victory of Protestant king William III of Orange against the ousted Catholic king James II at the Battle of the Boyne in 1690.
Throughout Tuesday there will be 573 Orange Order parades, 33 of which will pass through Catholic areas where tensions can simmer.
The celebrations were the busiest day of the year for police in Northern Ireland, plagued by three decades of sectarian bloodshed in British rule until a peace agreement in 1998.
About 2,500 officers will be on duty to deal with any violence, the Northern Ireland Police Service said.
Authorities treated an incident on Thursday in which petrol bombs and bricks were thrown at a bonfire site in north Belfast as a hate crime.
Police maintained a strong presence at the site.
The fire in Belfast’s Tiger Bay area has caused anger among pro-Irish nationalist residents living nearby, who say it is too close to their communities.
The months of the loyalist march in Northern Ireland leading up to July 12 were characterized by unionist opposition to the so -called Northern Ireland Protocol, which governs the province’s trade after Brexit.
The settlement, part of the UK’s divorce agreement with the EU, imposed checks on goods between Great Britain and Northern Ireland to avoid a difficult border crossing with the Republic of Ireland.
Proposed legislation introduced under Johnson to scrap parts of the protocol is currently underway in the UK parliament.
The Democratic Unionist Party of Northern Ireland refused to return to the provincial power -sharing government until the protocol was dissolved.
In March, paramilitary loyalists were blamed for a fraudulent bomb attack targeting visiting Irish Foreign Minister Simon Coveney.
Ireland and the EU in Brussels have accused London of violating international law on protocol law.