A mural showing Sinn Fein President Gerry Adams is pictured on the Falls Road in Belfast, Northern Ireland on May 3, 2014, as people gather to hear Northern Ireland's deputy first minister Martin McGuinness speaking during a rally to free Gerry Adams.
With tensions rising, his Sinn Fein party warned it could review its support for police in the British province, where thousands of people died during decades of Catholic-Protestant unrest.
The government rejected accusations that the arrest of Adams, the public face of the republican movement, was politically motivated.
Adams, 65, presented himself at a police station late Wednesday and was arrested for questioning over the murder of Jean McConville, a widowed mother of 10, in 1972.
McConville's children watched as she was dragged screaming from their home after the IRA accused her of being an informer. She was later found shot in the back of the head.
Detectives were on Friday granted an extra 48 hours until Sunday night to question Adams, after the previous deadline to charge or release him expired, the Police Service of Northern Ireland (PSNI) said.
A source close to Adams told the BBC that the political leader was being questioned "for 17 hours a day".
Senior members of Sinn Fein on Saturday led a group of around 400 supporters at the unveiling of a huge painted mural of Adams the "peacemaker" in his former constituency in the Catholic stronghold of Falls Road, Belfast.
Sinn Fein, the political wing of the now-defunct IRA, wants Northern Ireland to leave the United Kingdom and join the Republic of Ireland to the south. Adams played a leading role in the Northern Ireland peace process.
- 'Political policing' -
Deputy First Minister Martin McGuinness, the top Sinn Fein figure in the power-sharing government in Belfast, blamed "an embittered rump of the old RUC (Royal Ulster Constabulary)" for Adams' continued detention.
The widely-criticised RUC was the police force in Northern Ireland from 1922 to 2001.
"Allegations contained in books and newspaper articles which the PSNI are presenting to Gerry as evidence that he was in the IRA in the 1970s have been around for 40 years," he told the rally.
"But they are only now trying to use these. Is this not political policing?
"This is a replay of the failed effort in 1978 to charge Gerry with membership (of the IRA)."
"Sinn Fein's negotiations strategy succeeded in achieving new policing arrangements, but we always knew that there remained within the PSNI an embittered rump of the old RUC.
"These people want to settle old scores, whatever the political cost."
Sinn Fein assembly member Alex Maskey told the BBC Saturday the party would "monitor and review" its relationship with law enforcement services.
He said there was a "small element of people involved in policing who are politically motivated". The arrest came weeks before local and European elections.
"The people that I represent are scathing in their anger at the moment about the PSNI -- and it's not just about in relation to Gerry Adams," he said.
A PSNI spokesman said they had "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," he explained, adding the force was committed to treating "everyone equally before the law".
Republican support for the reformed Police Service of Northern Ireland (PSNI) -- which replaced the RUC -- was a key part of the peace process launched by the 1998 Good Friday agreement.
Catholic socialist Sinn Fein and the Protestant conservative DUP share power in an arrangement established under the Good Friday accords.
The accords largely brought an end to violence in Northern Ireland but sporadic attacks continue, blamed on dissident republicans opposed to the peace process, and communal unrest erupts from time to time.
British Prime Minister Prime Minister David Cameron has asked his senior security adviser to hold talks with Libya in order to try and obtain compensation for the families of republican attacks that used plastic explosive supplied by former Libyan leader Moammar Gadhafi, according to the Sunday Telegraph.
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