Gerry Adams will undergo a fourth day of questioning by detectives over the 1972 murder of Belfast mother-of-10 Jean McConville.
Extra time granted to police to question the Sinn Féin president will expire later today.
Amid a growing row over the arrest Northern Ireland’s Deputy First Minister said he believes it is part of an attempt by some police officers to settle old scores.
Martin McGuinness blamed an “embittered rump” left over from before the reform of policing whom he claimed wanted to destroy the peace process.
Addressing a rally in the staunchly republican Falls Road, he said: “These people want to settle old scores whatever the political cost.”
On Friday a judge allowed the PSNI a further 48 hours to hold Mr Adams at Antrim police station. The republican party has warned it will review its support for the police if the veteran leader is charged.
The 65-year-old detainee vehemently denies allegations levelled by former republican colleagues that he ordered Mrs McConville’s murder and secret burial more than 40 years ago after she was wrongly accused of passing information to the security forces.
Sinn Féin'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 said: “We worked very hard to reform policing.
“Sinn Féin's negotiations strategy succeeded in achieving new policing arrangements but we always knew that there remained with the PSNI an embittered rump of the old RUC (Royal Ulster Constabulary).
He added: “The arrest of Gerry Adams is evidence that there is an element within the PSNI who are against the peace process and who hate Gerry Adams and Sinn Féin.
“They are what the reformers within the PSNI have described to us as the dark side.
“They are small in number but very influential. This group is working to a negative and anti-peace process agenda and are actively involved in political policing.”
Stormont First Minister Peter Robinson, Justice Minister David Ford and Northern Ireland Secretary Theresa Villiers have defended the police’s handling of the arrest and denied the action was politically motivated and designed to undermine Sinn Féin ahead of the forthcoming European elections.
Mrs McConville was dragged, screaming, away 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 Adamsvoluntarily presented himself for interview by prior arrangement with detectives.
Mr McGuinness claimed 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.”
Stormont’s justice minister said Mr Adams’ arrest was “entirely appropriate”.
Mr Ford told the BBC Radio 4 Today programme on Saturday: “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.”
At yesterday’s rally several hundred people stood in silence, listening and clutching posters supporting the peace process. They included senior Sinn Féin figures Gerry Kelly, Caral Ni Chuilin, Jennifer McCann and European election candidate Martina Anderson.
In March Ivor Bell, 77, an IRA leader in the 1970s, was charged with aiding and abetting the murder of Mrs McConville.
The case against him is based on an interview he allegedly gave to researchers at Boston College in the US.
The Boston College tapes are a series of interviews with former loyalist and republican paramilitaries, designed to be an oral history of the Troubles.
The paramilitaries were told the tapes would only be made public after their deaths. However, after a series of court cases in the United States, some content has been handed to police.
At least one interviewee implicated Mr Adams in the murder of Mrs McConville but he has always strenuously denied any involvement.
A PSNI spokesman said: “Police have a duty to impartially investigate serious crime including murder. It is the police’s duty to make relevant inquiries, interview those with information, arrest and question suspects and, in consultation with the PPS, to either charge or submit a file to the PPS in relation to the investigation.
“This procedure is being followed in this case.
“As one individual has been charged with serious offences and files are being prepared in relation to other individuals, it would be inappropriate to comment further other than to reiterate the Police Service’s commitment to treat everyone equally before the law.”