A controversial Orange Order parade is set to go ahead on Sunday morning in a nationalist part of Northern Ireland for the first time in nearly 30 years.

Following a series of legal challenges, the High Court ruled in the early hours of Sunday that an initial decision by the Parades Commission to allow the Drumcree march to take place in Portadown had to be respected.

Hundreds of people have gathered on the Garvaghy Road ahead of the march, which can set off no later than 08:00 BST. Some are linking arms and singing songs. Police are monitoring the situation and dozens of armoured vehicles have arrived in the town.

The Drumcree parade, which in the 1990s sparked some of the most serious disorder in Northern Ireland’s history, had been given permission to go ahead along the Garvaghy Road with restrictions in place.

However that decision was challenged in several Belfast courts on Saturday, before finally being given the go ahead just hours before it is due to take place.

Police officers are operating checkpoints on the main roads into Portadown, and they are reporting a steady stream of vehicles arriving into the County Armagh town.

Drivers are being handed an information sheet, which says: “You are entitled to disagree with the procession and to make your views known peacefully.

“The procession has been permitted by a determination of the Parades Commission. People taking part in it also have a right to do so peacefully and safety.”

It says police will seek to facilitate peaceful protest where possible.

In a hearing concluding at 02:15 BST on Sunday morning, Justice Michael Humphreys acknowledged the “unusual if not unprecedented circumstances… with a parade due to begin in just over six hours’ time”.

Delivering his judgement, Justice Humphreys said: “This court cannot substitute its own view or anyone else’s view for that of the commission.”

Northern Ireland’s most senior judge, Lady Chief Justice Siobhan Keegan, similarly described the situation as “unprecedented”.