BELFAST/DUBLIN (Reuters) - Northern Ireland police arrested Sinn Fein leader Gerry Adams on Wednesday as part of an investigation into one of the province's most controversial murders, a move likely to cause a political earthquake in Belfast and Dublin.

The man reviled in Britain as the spokesman for the Irish Republican Army in the 1980s, Adams reinvented himself as a Northern Ireland peacemaker and then as a populist opposition politician in the Irish parliament.

His Sinn Fein party on Wednesday said he was being questioned by police investigating the 1972 abduction and murder of mother of 10 Jean McConville. The police said a 65-year-old had been arrested by detectives investigating the killing.

Adams, who has always denied membership of the IRA said he was "innocent of any part" in the killing.

"I believe that the killing of Jean McConville and the secret burial of her body was wrong and a grievous injustice to her and her family," Adams said in a statement.

"Well publicized, malicious allegations have been made against me. I reject these," he said.

BODY ON BEACH

McConville's body was found in 2003 on a beach in county Louth, which Adams now represents in Ireland's parliament. McConville had been suspected by the IRA being an informer, a charge her family has always denied.

The investigation into McConville's killing has been revived by the release of a series of interviews given by former fighters from the Northern Ireland conflict to Boston College.

The Northern Ireland police service last year asked for taped testimony from former IRA bomber Dolours Price following her death last year.

As head of the political wing of the Irish Republican Army, Sinn Fein, Adams was a pariah in 1980s Britain, banned from speaking on British television.

He later helped broker a 1998 peace deal that largely ended three decades of violence between Catholic militants seeking union with Ireland and mainly Protestant militants, who wanted to maintain Northern Ireland's position as a part of Britain.

Since that peace deal Adam's role as a statesman has grown. He is a regular visitor to the White House and was part of a guest of honor at the funeral of Former South African president and anti-apartheid hero Nelson Mandela last year.

FRAGILE PEACE

Northern Ireland's fragile peace has been shaken by investigations into historic crimes in recent years, with probes into pro-British militants widely seen as one of the sparks for some of the 2013 street violence that was the worst for years in the province.

It is unclear what affect the arrest might have on Northern Ireland's power-sharing government, whose deputy first minister, former IRA commander Martin McGuinness, is also a member of Sinn Fein who has reinvented himself as a statesman, meeting the British Queen in 2012.

The arrest will have major ramifications in the Republic of Ireland where Adams leads the second largest opposition party Sinn Fein, campaigning on opposition to the government's austerity policies.

An image makeover to make him more palatable to a public where suspicion of Sinn Fein's role in the Northern Ireland troubles runs deep has included a twitter feed that recounts the escapades of Adam's teddy bears and Pilates classes.

He was forced to disassociate himsef from his brother Liam who was sentenced last year to 16 years in prison for raping his daughter when she was a child. The Public Prosecution Service decided not to prosecute Gerry over allegations of withholding information from the police on the issue.

Adams, who is campaigning for Sinn Fein candidates in European elections on May 23, suggested his arrest could be politically motivated.

"I do have concerns in the middle of an election about the timing," he told Irish television station RTE before he arrived for questioning.

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