Paradegoers carried the orange collarette, a V-shaped sash worn around the neck. Many also came in the stern-looking dark sunglasses, black bowler caps, and white gloves that have become synonymous with the marches.
At least two women chose a more relaxed uniform: Red, white, and blue wigs and Union Jack dresses.
The loud and colorful marches date to the 19th century and are a long-standing irritant between Northern Ireland's two main religious communities. The loyal orders see them as expressions of their culture and a testament to their faith. Many Catholics see them as aggressive and anti-Irish, and the marches can devolve into street fights, particularly when they pass through heavily Catholic areas.
There were no immediate reports of any unrest, and marchers passed a potential flashpoint on the parade route - St. Patrick's Church near Belfast's city center - without incident.
"They marched with dignity down the road," said the Rev. Michael Sheehan, administrator of St Patrick's. "I think a degree of respect was shown that hasn't been shown before."
Nearly every aspect of the marches - from the parade route to the music played - is argued over and litigated by both sides, and a specially created Parades Commission mediates between the two. The commission can, for example, reroute the parade around a potential flashpoint or demand that sectarian songs not be sung in certain areas.
The marches are steeped in Northern Ireland's messy history, and Saturday's massive parade was expected to draw up to 30,000 people. The march was due to finish at Stormont with a cultural festival commemorating a 1912 proclamation against plans for home rule in Ireland.
Crowd estimates were not immediately available.