The PSNI has been granted an additional 48 hours to continue questioning Gerry Adams in relation to the abduction and murder of mother-of-ten Jean McConville.
The Sinn Féin President and Louth TD has been in police custody since he turned up by appointment at Antrim PSNI station on Wednesday and was arrested.
Mr Adams has always vehemently denied allegations by former republican colleagues that he ordered the murder of Mrs McConville, who was wrongly suspected of being an informer to the British army.
Northern Ireland Deputy First Minister Martin McGuinness has accused the British government and elements of the PSNI of "pursuing a negative and destructive political agenda".
He said: "Yesterday I said that the timing of the arrest of Gerry Adams was politically-motivated.
"Today's decision by the PSNI to seek an extension confirms me in my view."
Mr McGuinness said there were many progressive and open-minded elements in the new policing arrangements, which he supported.
But he also claimed there is a "small cabal" in the PSNI who have a negative and destructive agenda to both the peace process and to Sinn Féin.
Mr McGuinness said Sinn Féin would "reflect" and "review" its support for policing in the region if Mr Adams is charged.
But he urged republicans to remain calm if and until that happened.
Sinn Féin MLA Gerry Kelly said: "The arrest and continued detention of Gerry Adams is deliberately timed to coincide with the elections in three weeks time."
He said: "This is political policing at its most blatant.
"Sinn Féin will not be intimidated by the action of a small cabal in the PSNI who are opposed to the peace process and political change." He said.
Earlier, the Government took issue with a claim made by Mr McGuinness about the arrest of Mr Adams.
Taoiseach Enda Kenny said Mr McGuinness's comments are a contradiction of the statements that Sinn Féin has made about the progress the PSNI has been making over the past number of years.
Mr McGuinness said yesterday that the arrest of Mr Adams in connection with the abduction and murder of Jean McConville was aimed at inflicting political damage on Sinn Féin.
He also claimed that the arrest was an example of the "dark side" of policing trying to flex its muscles.
Mr Kenny said the real victims were the ten children of Mrs McConville and that the PSNI was not the RUC.
The Taoiseach said comments by Mrs McConville's son Michael that he would be putting his and his family's lives at risk if he named his mother's killers were of concern.
He said everybody who had information about the murder had a duty and a responsibility to give it to the PSNI.
Tánaiste and Minister for Foreign Affairs Eamon Gilmore described the questioning of Mr Adams as a very significant event.
He said different rules do not and should not be applied to politicians in murder investigations.
Mr Gilmore said distinctions should not be drawn in police investigations because an individual happens to be a public representative.
Officers resumed questioning the 65-year-old politician about the 1972 killing this morning.
McConville daughter prepared to give names
Mrs McConville's eldest daughter last night said she is prepared to name the people she believes are responsible for her mother's death.
More than 40 years after her mother was abducted from her flat in west Belfast, Helen McKendry said she no longer feared reprisals by republicans.
"What are they going to do to me? They have done so much to me in the last 42 years," she told BBC's Newsnight.
"Are they going to come and put a bullet in my head? Well, they know where I live."
Mrs McKendry's declared willingness to speak out came after her brother Michael said he was still not prepared to name those involved even though he knew who they were.
He claimed he was too scared to tell detectives who he believed was responsible in case he or other family members were shot by republican extremists for informing.
"Everybody thinks that the IRA has gone away but they have not. If we tell we will be shot," he said.
Mrs McKendry, who was 15 at the time, said she was convinced that Mr Adams was involved, despite his repeated denials over the years.
"I have always believed that Gerry Adams was involved in the murder of my mother. Until the day I die, I will believe that," she said.
No one has ever been charged with the murder of the 37-year-old widow, but after years without progress in the criminal investigation there have been a series of arrests in recent weeks.
Veteran republican Ivor Bell, 77, was charged in March with aiding and abetting the murder.
Five other people have been detained and questioned.
Mrs McConville was dragged from her children in the Divis flats in west Belfast by a gang of up to 12 men and women.
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.
Villiers says police impartial
Secretary of State for Northern Ireland Theresa Villiers has said police have operated with impartiality.
Ms Villiers urged political leaders to give measured responses.
She said: "I believe that the PSNI take their duties in terms of impartiality, balance and respect for human rights extremely seriously.
"They are not a politicised police force, they are properly and highly scrutinised by the Policing Board and Police Ombudsman."
Earlier, DUP MP Gregory Campbell said due process needs to be allowed to take its course in relation to the arrest of Mr Adams.
Speaking on RTÉ's Morning Ireland, Mr Campbell rejected claims that the timing of the arrest was politically motivated.
SDLP Leader Alistair McDonnell responded to the Sinn Féin claims by saying it could be equally be argued that it would have been much more political of the PSNI not to act.
He told the same programme that it was a live investigation and it was essential that it be pursued.