Martin McGuinness has reiterated his belief that the arrest of Sinn Féin president Gerry Adams was politically motivated, accusing members of the PSNI of being against the peace process.
Mr McGuinness, Northern Ireland's deputy first minister, said there was an "embittered rump of the old RUC (Royal Ulster Constabulary)" within the PSNI that was trying to settle old scores.
He spoke at the unveiling of a mural in Belfast of Mr Adams, who is spending his fourth day in police custody in connection with the murder of mother-of-10 Jean McConville in 1972.
Mr McGuinness told the cheering crowd that Mr Adams' arrest was designed to affect Sinn Féin ahead of local and European elections.
He said: "No police force anywhere in the world is immune from criticism if it is acting in a politically biased and partisan fashion.
"The arrest of Gerry Adams is evidence of that fact that there is an element in the PSNI who are against the peace process and hate Gerry Adams and hate Sinn Féin.
"They are what the reformers within the PSNI have described to us as the 'Dark Side'."
Last night a judge allowed the PSNI a further 48 hours to hold Mr Adams at Antrim police station.
Sinn Féin has warned it will review its support for the police if the veteran leader is charged.
Mr Adams, 65, vehemently denies allegations levelled by former republican colleagues that he ordered Jean McConville's murder and secret burial more than 40 years ago after she was wrongly accused of passing information to the security forces.
Alex Attwood of the SDLP described the remarks by Mr McGuinness, as a challenge to Irish democracy.
The West Belfast MLA said questions were raised by the fact that the Sinn Féin rally was held in the shadow of Divis, from where Jean McConville was taken in 1972, and then murdered.
Mrs McConville was dragged from her children in the Divis flats in west Belfast by a gang of up to 12 men and women in 1972.
She was interrogated, shot in the back of the head and then secretly buried. She became one of the "Disappeared" victims of the Troubles.
Her body was not found until 2003, on a beach in Co Louth, 80km from her home.
The British Prime Minister has urged the leaders of Northern Ireland's power-sharing government to co-operate after the arrest prompted sharp divisions between them.
David Cameron spoke to DUP First Minister Peter Robinson and Mr McGuinness, amid heightened republican anger at the timing of the Mr Adams's detention.
DUP leader Mr Robinson said it would have been political policing if the PSNI had decided not to investigate Mr Adams because of the pending poll.