Martin McGuinness has reiterated his belief that the arrest of Sinn Fein president Gerry Adams was politically motivated, accusing members of the PSNI of being against the peace process.
McGuinness, Northern Ireland's deputy first minister, said there was an "embittered rump of the old RUC" within the PSNI that was trying to settle old scores.
He spoke at the unveiling of a mural on the Falls Road in west Belfast of 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 Adams' arrest was designed to affect Sinn Fein 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 Fein.
"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 the Sinn Fein president at Antrim police station. The republican party 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.
Sinn Fein's decision to sign up to support the police in 2007 was viewed as a major milestone in the peace process and prompted the return to devolved rule at Stormont, with the republican party and the Democratic Unionists entering government together.
Mr McGuinness added today that the PSNI was guilty of "political policing", and using information that had been around for 40 years.
He said: "In my view this is a failed attempt at the replay of the effort in 1978 to charge Gerry Adams with membership of the IRA. That case was based on hearsay, gossip and newspaper articles. It failed then and it will fail now.
"Thirty six years later those within the PSNI who are hostile to the peace process are using the same old dirty tricks. They are deliberately and cynically exploiting the awful killing of Jean McConville and the grief and hurt inflicted on her family."
Mrs McConville was dragged screaming from her children in the Divis flats in west Belfast by a gang of up to 12 men and women after being wrongly accused of informing to the security forces.
She was interrogated, shot in the back of the head and then secretly buried - becoming one of the "Disappeared" victims of the Troubles. Her body was not found until 2003 on a beach in Co Louth, 50 miles from her home.
Mr Adams, a former MP for West Belfast and now an elected representative for Co Louth in the Irish Dail, voluntarily presented himself for interview at Antrim PSNI station by prior arrangement with detectives.
Stormont's justice minister David Ford said this morning that Adams' arrest was "entirely appropriate".
He told the BBC's Today programme: "It is normal practice if somebody is likely to be arrested in the course of an inquiry that they are arrested at the start of discussions."
The justice minister added: "I don't know whether Gerry Adams thought he was going to turn up at Antrim's serious crime suite, have a wee chat for half an hour and then go off again, but clearly on the scale of the concerns expressed, of the information - which I entirely accept is not yet evidence - it was entirely appropriate that should be followed up in the normal way.
"Those decisions are for the police, supported yesterday by an independent judge in extending the time for that investigation to continue."