* First Minister Robinson says Sinn Fein have crossed a line
* Sinn Fein say peace process, government not under threat
* Police have until 1900 GMT to release, charge or extend (Adds McGuinness response, details of Adams questioning)
By Conor Humphries
ANTRIM, Northern Ireland, May 4 (Reuters) - Northern Ireland's first minister Peter Robinson on Sunday accused Sinn Fein, his partner in government, of a "thuggish attempt to blackmail" police through its criticism of the arrest of its party leader Gerry Adams.
Irish and British media reports said that Adams, who has been questioned by Northern Ireland police since last week over a 1972 murder, would be freed later on Sunday and a file sent to the public prosecutor's office.
Detectives, who have been interrogating Adams since Wednesday, must decide by 1900 GMT whether to charge, release him, or seek a further extension in custody.
The arrest of Adams has rocked the province.
Nationalist Sinn Fein, which shares power with Robinson's Protestant Democratic Unionist Party (DUP), has said the arrest is a deliberate attempt by "dark forces" in the Police Service of Northern Ireland (PSNI) to undermine the peace process.
Deputy first minister Martin McGuinness stopped short of saying Sinn Fein would remove its support for the PSNI, a move that would spark a major crisis. But he said on Friday that it would wait to see if the situation was resolved satisfactorily.
"The publicly conveyed threat to the PSNI delivered by the highest levels of Sinn Fein that they will reassess their attitude to policing if Gerry Adams is charged is a despicable, thuggish attempt to blackmail the PSNI," Robinson said in a statement.
"The threat now means that ordinary decent citizens will conclude that the PSNI and the PPS (Public Prosecution Service) have succumbed to a crude and overt political threat if Adams is not charged.
"I warn Sinn Fein that they have crossed the line and should immediately cease this destructive behaviour."
Responding in Dublin, McGuinness said he unequivocally supported the PSNI but had a problem with a tiny number of people in influential positions opposed to Sinn Fein who saw the arrest of Adams as "payback time".
He added that the peace process was not at risk over the crisis, nor was the power-sharing government under threat from Sinn Fein. Northern Ireland Justice Minister David Ford urged both side to take a step back and let the police to their job.
Adams' arrest over the killing of Jean McConville is among the most significant in Northern Ireland since a 1998 peace deal ended decades of tit-for-tat killings between Irish Catholic nationalists and mostly Protestant pro-British loyalists.
The Sinn Fein leader, who is a member of parliament in the Irish republic, has been dogged throughout his career by accusations from former IRA fighters that he was involved in its campaign of killings, a charge he has repeatedly denied.
When he was arrested, Adams said that he was "innocent of any part" in the killing, which he said was "wrong and a grievous injustice to her and her family".
Gerry Kelly, a Sinn Fein member of the Northern Ireland Assembly who on Sunday visited Adams and his legal team at the police station where he is being held, told reporters that Sinn Fein had not been told what was going to happen later on Sunday.
Two dozen pro-British activists raised a union flag from a lamp post near the police station. However, there have been no signs of trouble across the province since Adams' arrest.
Kelly said that according to Adams' solicitor, the former leader of the IRA's political wing was being quizzed on decades- old photographs and newspaper articles, a tactic McGuinness described as a trawling exercise similar to the 1970s when attempts were made to charge Adams with IRA membership.
Unlike McGuinness, Adams, who helped broker a 1998 peace deal that largely ended three decades of sectarian violence, has always denied membership of the IRA. (Additional reporting by Padraic Halpin in Dublin; Editing by Sophie Hares and Stephen Powell)