* Sinn Fein leader released, file sent to public prosecutor
* Adams says determined to keep peace, supports police
* First Minister Robinson says Sinn Fein have crossed a line
* Protestors throw petrol bombs in Belfast, no injuries (Adds petrol bombs thrown in Belfast, no injuries.)
By Conor Humphries
BELFAST, May 4 (Reuters) - Northern Ireland police released Gerry Adams from custody on Sunday and the Sinn Fein leader sought to calm fears that his four-day detention could destabilise the British province by pledging his support to the peace process.
Police arrested Adams on Wednesday over the 1972 murder of Jean McConville, a killing he repeated that he was "innocent of any part" in. His detention had raised tensions among Northern Ireland's power-sharing government and its fragile peace.
After Sinn Fein pointed the finger at "dark forces" in the police service and their Protestant partners in government accused it of a "thuggish attempt" at blackmail, a calm Adams toned down the rhetoric and said he supported the police.
"My resolve remains as strong as ever, that is to build the peace, not to let this put us off. It's our future. The past is the past," Adams told a news conference attended by about 150 cheering supporters in a hotel in west Belfast.
"The old guard which is against change, whether it is in the PSNI (Police Service of Northern Ireland) leadership, within elements of Unionism or the far fringes of self-proclaimed, pseudo-republicans, they can't win."
"I'm an Irish republican. I want to live in a peaceful Ireland. I've never dissociated myself from the IRA and I never will, but I am glad that I and others have created a peaceful and democratic way forward for everyone. The IRA is gone, finished."
Adams' arrest over the killing of McConville was among the most significant in Northern Ireland since a 1998 peace deal ended decades of tit-for-tat killings by Irish Catholic nationalists and mostly Protestant pro-British loyalists.
In the predominantly Protestant Sandy Row area of Belfast, police said they had to deal with some disorder when a number of petrol bombs and stones were thrown. Nobody was believed to have been injured, a police spokeswoman said.
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.
McConville, who was accused by the IRA of being an informer for the British, an allegation her family has always denied, was dragged away in front of her children, one of 15 people living in strongly republican areas who were spirited away by the IRA and dumped in unmarked graves.
The killing has haunted Adams and has been raised repeatedly in interviews during his transformation from the face of Irish militant nationalism to mild-mannered politician. Police said it would now send a file to the public prosecutor.
Under the 1998 Good Friday agreement, which drew a line under 30 years of sectarian strife in the British province, those convicted of paramilitary murders during the conflict would have life sentences reduced to two years.
Adams said he did 33 taped interviews during his time in the Antrim prison and that police offered no evidence against him, only allegations based on a mishmash of decades-old newspaper articles, photographs and the series of taped interviews given by former guerrillas for a research project in the United States that revived the investigation into McConville's killing.
Asked what it was like spending four nights in prison, a stony-faced Adams replied: "It was OK". He added that the food was inedible and the facility not up to modern standards.
He also reiterated Sinn Fein's belief that his arrest was timed to hurt its chance in European and local elections on both sides of the border later this month, as the party gains popularity in the Irish republic amid the financial crisis.
Adams told the news conference broadcast live across Irish and British television that the decision by police to detain him was sending the entirely wrong signal to those who do not want the peace process to work.
The arrest was particularly controversial as it raises questions about two cornerstones of the peace deal that has transformed Northern Ireland: the pardoning of militants, and the confidence of all sides in the neutrality of the police.
At the heart of the stand-off is the fact the 1998 deal had neither a blanket amnesty nor the kind of exhaustive Peace and Reconciliation Commission that lifted the threat of prosecution from South Africans who confessed to apartheid-era crimes.
NO SIGN OF TROUBLE
First Minister Peter Robinson, whose Protestant Democratic Unionist Party (DUP) share power with nationalist Sinn Fein, earlier raised the stakes when he said his partner in government had crossed a line through its criticism of the arrest.
"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," he said in a statement.
"The threat now means that ordinary decent citizens will conclude that the PSNI and the PPS have succumbed to a crude and overt political threat if Adams is not charged. I warn Sinn Fein that they have crossed the line."
Northern Ireland Justice Minister David Ford urged both sides to take a step back and let the police do their job. Ireland's Deputy Prime Minister Eamon Gilmore called on all parties to respect this process and to refrain from further comment.
The investigation of former militants on both sides of the conflict has stirred protests in the province in recent years.
Some 100 pro-British activists protested outside the Antrim police station where Adams was held, forcing police to send armoured vehicles and officers in riot gear out the front gate to divert attention as Adams slipped out the back. (Additional reporting by Maurice Neill and Padraic Halpin in Dublin; Writing by Padraic Halpin; Editing by Jon Boyle, Eric Walsh and Paul Simao)