Martin McGuinness has challenged political rivals making claims about Sinn Fein links to criminality to "put up or shut up".
As cross-party talks got under way in Belfast to save the power sharing administration amid a crisis over an IRA-linked murder, the Sinn Fein Deputy First Minister made a fiery defence of his party's commitment to peace.
"I am not putting up with this nonsense any more," he said.
However, Democratic Unionist leader Peter Robinson, who took part in the talks less than 24 hours on from being discharged from hospital after suffering a bad reaction to heart medication, said it was vital the process examined paramilitary links with "political parties in government".
The devolved Assembly has been thrown into disarray following the murder of ex-IRA man Kevin McGuigan last month.
The 53-year-old father of nine was shot dead in Belfast in a suspected revenge attack for the murder of former IRA commander Gerard "Jock" Davison, 47, three months earlier. Detectives believe some of Mr Davison's associates suspected Mr McGuigan of involvement in his shooting.
A police assessment that individual members of the IRA were involved in the McGuigan murder prompted unionists to remove all ministers but one from the coalition Executive, claiming Sinn Fein was inextricably linked to the supposedly defunct republican terror group.
Mr McGuinness had strong words for his party's accusers as he emerged from the first round table talks session with the four other Executive parties and the British and Irish governments at Stormont House.
"Some parties within these negotiations are attempting to give the impression that there's linkage between a party in government - what they mean by that is Sinn Fein - and criminality," he said.
"During the course of this morning's engagement I took those people to task. I told them if anybody has any information whatsoever about any party in government being involved in criminality that they should put it up on the table and, more importantly, they should put it to the PSNI (Police Service of Northern Ireland).
"In other words I told them it is now time for them to put up or shut up."
Mr McGuinness name-checked the Ulster Unionists and SDLP in the course of his comments.
"I am not going to be accused by others who are trying to besmirch the name of my party," he said.
Last week, the DUP and UUP said the issue of paramilitary activity was crucial to their participation in any talks process.
In response, on Friday the UK Government announced an independent assessment of paramilitary criminality and pledged increased funding to tackle cross border organised crime such as fuel laundering and smuggling.
No major breakthrough is expected in the talks process until the paramilitary assessment is published, which is expected in mid-October.
Mr Robinson said there was a need for a full examination of MI5, National Crime Agency (NCA) and PSNI material on the status of the IRA.
"It's necessary for us to have that because part of the issues that we will be dealing with, in terms of paramilitaries, relates to links with political parties," he said.
Mr Robinson, who has stood aside as First Minister during the crisis, said his party had six key proposals, including the establishment of a "permanent" structure to provide a comprehensive examination and assessment of paramilitary activity.
"One of our six demands, and it will be no surprise, will be for a permanent structure to carry out that kind of review and, like the IMC (International Monitoring Commission), it should have sanctions applied to anybody who breaches it," he said.
Prior to the McGuigan murder, the future viability of the administration had already been in doubt as a consequence of long-standing budgetary disputes, with the row over the non-implementation of the UK Government's welfare reforms the most vexed.
The fallout from the shooting and the other problems besetting power-sharing are all on the agenda in the negotiations.
Ahead of the talks, Northern Ireland Secretary Theresa Villiers warned politicians not to waste the opportunity to build a better future.
Mr Robinson was admitted to hospital on Saturday after suffering an adverse reaction to medication prescribed following his heart attack in May.
At Stormont today he said he was feeling fine.
"As someone who left it 66 years to get into hospital in the first place, I am trying to make up for it a bit," he said.
"It was simply a bad reaction to some of the medication. It has been corrected and I am feeling much better already."
At the close of the first day of talks, Ms Villiers described the exchanges as "focused and productive".
"All parties committed themselves to an intensive process aimed at dealing with the impact and legacy of continued paramilitary activity and securing the full implementation of the Stormont House Agreement," she said.
"Tomorrow the talks will concentrate on finance and welfare issues. Implementing these aspects of the Stormont House Agreement is essential if the Executive is to have a workable and sustainable budget so that it can continue to deliver essential frontline public services."