After several hours of stalemate when the Royal Ulster Constabulary stopped an Orange parade from marching through a Catholic area, several hundred more Protestants joined the demonstration.
In response, the RUC drafted in extra riot police as tension grew after all entrances to the centre of the town were closed off to prevent the march.
Hundreds of officers used Land Rovers to close the roads while republicans blocked the Garvaghy road along which the Orangemen traditionally parade after their annual service at Drumcree church, outside town.
About 800 Orangemen remained outside the church hoping they might be allowed to march. Yesterday's parade - in the bicentenary year of the formation of the Orange order - was the first time the march has not proceeded as planned.
The Upper Bann Ulster Unionist MP David Trimble - a member of the Orange Order - said: "We are angry at the failure of the police to secure the route for a peaceful parade. They have made absolutely no effort to do so."
The Co Armagh Grand Orange Lodge issued a statement calling on Orangemen throughout Northern Ireland to "urgently muster at Drumcree church to show their solidarity with their Portadown brethren".
By late evening, the assembly of Orangemen and supporters had grown to more than 5,000 with many more arriving from various centres. The Rev Ian Paisley, the DUP leader, spent more than 40 minutes discussing the situation with RUC chiefs. As news spread, there were protests in other towns and villages, and the tension looked set to last through the night.
tHugh Lewsley, a Social Democratic and Labour Party councillor in Lisburn who has been an outspoken critic of the republican movement, was treated in hospital for injuries he says were inflicted in an IRA punishment beating.Reuse content