Peter Robinson has bluntly challenged Sinn Fein MLAs to admit the IRA was involved in "acts of cowardice" – by a show of hands in the Assembly chamber.
The First Minister directly asked Martin McGuinness and his colleagues to put their hands up in an extraordinary attack responding to criticism of unionist leaders last week.
The angry DUP leader also accused the Deputy First Minister of attempting to deflect attention and making a "synthetic" attack on unionist leaders.
As murmurs of protest on the Sinn Fein benches grew, Speaker William Hay had to intervene to restore order. But later, outside the chamber, Mr McGuinness hit back, warning that in retreating into the past, Mr Robinson had "entirely missed the point".
It was a further signal of the deteriorating relationship between Stormont's 'top two' although tensions between their two parties have also increased in the run-up to this week's double European and council elections.
Mr McGuinness' broadside last week – when he accused unionist leaders of failing to join him in condemning racist attacks for which he blamed the UVF and "elements" of the Orange Order – was raised during question time yesterday by the DUP's Jonathan Craig.
Looking across at Mr McGuinness, Mr Robinson referred to "cowardice of the worse kind – those who shoot people in the back and have done so in the past."
He argued Mr McGuinness's remarks had been designed to deflect attention from his qualified support for policing arrangements which was threatened in the aftermath of Gerry Adams arrest in connection with the 1972 murder of Jean McConville.
Mr Robinson went on: "It's the cowardice of those who take out a young woman, in the midst of her family, a widow with 10 children, take her away and torture her, tie her hands behind her back and then shoot her in the back of the head – that's cowardice of the worst kind," he said. And he then urged Sinn Fein MLAs to raise their hands if they believed the IRA had exhibited cowardice – but none of them did so.
"Not one," Mr Robinson continued. "Not one would say that those who tied a bomb to the window of the La Mon Hotel and then had a napalm-style effect on those who had gone to enjoy a dinner for the Collie Club – that is cowardice of the worst kind. Those who planted a bomb at a Remembrance Service in Enniskillen – that's cowardice of the worst kind.
In a statement later Mr Mr McGuinness said he had tried to avoid recriminations about the past even though he could raise many issues including unionist politicians' involvement in the Ulster Workers strike, collusion with loyalist paramilitaries and setting up Ulster Resistance.
"I am talking about the here and now – the failure of unionist leaders to stand shoulder to shoulder with the rest of us in denouncing racist violence," he said.