Party president Gerry Adams dismissed the IMC report as a sham and as a "proxy report by securocrats".
At a press conference in Dublin yesterday evening, Mr Adams dismissed the independence of the commission and also criticised what he said was its bias against republicans. He said it "reduced the ongoing unionist paramilitary campaign almost to a postscript".
Mr Adams said he would seek urgent discussions with Taoiseach Bertie Ahern and British Prime Minister Tony Blair on the report and accused the IMC of relying on British security services for its information. He questioned how the commission had managed to produce such conclusive findings after only three months, comparing it unfavourably to shabby treatment meted out to the family of Pat Finucane.
"Sinn Féin will not accept this partisan report. We will not accept this attack on our party," he said.
He said Sinn Féin would challenge its findings by every means at its disposal.
Mr Adams said the Irish Government had repeatedly failed to uphold its obligations under Good Friday Agreement since 2001 and had acquiesced to the agenda of the British government. "Since the Assembly elections last November, the Irish Government has led the charge in a reckless way, driven by the upcoming elections, in a vicious propaganda offensive."
He went on to openly accuse the Government's position on the forthcoming citizenship referendum as "racist".
Mr Adams would not take questions at the press conference, which was attended by a large number of Sinn Féin supporters.
However, when asked on RTÉ's Six-One News about the key finding linking senior members of Sinn Féin with the IRA, he accepted people had a right to know of such links if they existed.
When asked if members of Sinn Féin's Árd Chomhairle were currently in the IRA, he replied: "Well not to my knowledge."
When pressed, he added: "I wouldn't necessarily know if they were."