Organisers said a counter-protest in Scarva, County Down, was "racist, xenophobic, sectarian".
Carla Lockhart responded to criticism by saying she was there to stand with her "community" and to prevent "tensions escalating".
Dozens of police officers, some in riot gear, and water cannons are in the village of Scarva.
Connor Phillips and Cate Conway completed a 25 mile walk from Portadown to Newry which began on Tuesday.
Four men have been arrested after minor disorder associated with a protest held in Main Street in Scarva.
The event features a parade and a reenactment of King William's victory over King James in 1690.
Tens of thousands gather in County Down for the annual "battle" between King William and King James.
The Royal Black Institution says the Scarva event could not be held in accordance with guidance.
