The bombing meant that a session of multi-party talks was once again eclipsed by events on the streets, demonstrating that groups opposed to the process are intent on using violence to sabotage it.
While Unionists immediately claimed that the IRA must have been responsible for the bombing, the authorities said they would await police reports before attributing blame. Last night, the IRA indicated it was not responsible. A brief statement, telephoned to the Dublin newsroom of Ireland's RTE national broadcaster, made no direct reference to the bomb but said: "We reiterate that the complete cessation of military operations, which began at midday on Sunday July 20 last year, remains intact."
Either way the bombers were clearly aiming for maximum political and communal disruption in choosing as their target the Co Armagh town of Portadown, which is sometimes called the Orange citadel.
It is both the home town of the Loyalist Volunteer Force, currently the most violent of the Protestant paramilitary groups, and the central town of Ulster Unionist leader David Trimble's Upper Bann constituency. It is also the venue for the annual Drumcree marching confrontation in July.
It is thus one of the most provocative targets that republicans could choose to bomb. Mark Fulton, a close associate of assassinated LVF leader Billy Wright, said: "The people in Portadown are incensed. I would say loyalists will not be amused at this."
Warnings had been given and no one was injured when a large car-bomb was set off in the centre of the town, wrecking many business premises and damaging buildings over a wide area.
The attack brought a warning from Billy Hutchinson, of the Progressive Unionist Party, which speaks for the Ulster Volunteer Force. The UVF, while no saints, have been the quietest of the loyalist paramilitary outfits and the most supportive of the peace process. Mr Hutchinson said: "The loyalist ceasefire is under threat if these attacks continue. Political dialogue is not working. Time is running out. I think the North Armagh brigade of the Provisional IRA are at their work."
His allegation was echoed by other Unionists, including Mr Trimble, who said: "I have no doubt that this is the work of the IRA. We are seeing Sinn Fein-IRA venting its spleen because it has been caught out. It has been exposed inside the process and outside the process. There can be no question now of their returning to the talks."
He strongly appealed to loyalists not to retaliate, adding: "The spotlight is on the republican movement. They're in the dock, there's no reason why any loyalist should go and join them in the dock."
The security forces have yet to reach a conclusion on the allegation of IRA involvement. Unionist politicians have often made similar claims in the wake of other bombings which have turned out to be the work of the Continuity Army Council, whose members are renegade breakaway republicans opposed to the IRA.
This was the case following last Friday's car-bomb in the town of Moira, Co Down, which security sources now attribute to the CAC rather than the IRA. But the frequent Unionist claims that CAC equals IRA mean that many members of the public, especially on the Unionist side, find it difficult to differentiate.
The confusion is of some political use to Unionists who are campaigning to have Sinn Fein excluded from the multi-party talks not just for two weeks but for good.
The LVF was also active yesterday, leaving an explosive device outside a border police station in the Irish Republic. The device, which was dealt with by army experts, contained two five-gallon drums of petrol, a biscuit tin of 2.5kg of commercial explosive, a home-made timing unit and detonators, as well as 40 rounds of pistol ammunition.
This week the Irish Prime Minister, Bertie Ahern, is to meet Tony Blair and the Sinn Fein president, Gerry Adams for talks.
Mr Blair has not yet said whether he will react to Sinn Fein's request for a meeting with him.