Northern Ireland's Deputy First Minister has blamed the "dark side" of policing conspiring with enemies of the peace process for the arrest of Gerry Adams, the Sinn Fein president.

Martin McGuinness acknowledged yesterday that Jean McConville was the victim of a terrible wrong done by the IRA but said Wednesday's detention in Antrim was a deliberate attempt to influence the outcome of European elections due in three weeks' time.

His allegations were denied by Prime Minister David Cameron, who said: "There has been no political interference in this issue."

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Matt Baggott, the chief constable of the Police Service of Northern Ireland would not comment in detail on Mr Adams' arrest. But he said the McConville investigation would be "effective, objective and methodical".

The claims and counter claims emerged on a day that one of Mrs McConville's sons revealed he was too scared to tell detectives who he believed was responsible.

Mr McGuinness, the head of Sinn Fein's team at the devolved Northern Ireland Assembly, has condemned the shooting dead of police officers by dissident republicans opposed to the peace process.

But the arrest of Mr Adams, a former Westminster MP and current Irish Parliament member in the midst of an election campaign has angered republicans who hope to make major vote gains in the Irish Republic.

Mr McGuinness said: "I think we have seen that dark side flex its muscles in the last couple of days."

His partner at the head of the Stormont coalition, First Minister Peter Robinson, said it would have been political policing had the PSNI decided not to investigate Mr Adams.

The Democratic Unionist Party (DUP) leader said: "It strengthens our political process in Northern Ireland for people to know that no one is above the law, everyone is equal under the law and everyone is equally subject to the law."

Mr McGuinness said serious questions had to be asked of the agenda of those behind the arrest.

"In the mouth of an election the leader of a political party experiencing huge growth all around the island finds himself under arrest."

He claimed Sinn Fein had been told by "senior" and "reforming" figures within the Police Service of Northern Ireland that there was still a dark side within policing.

He said some former republicans who were "maliciously and vehemently" hostile to the peace process had been targeting Mr Adams. "It is quite disappointing to see the efforts of those people now in consort with the dark side within policing."

Asked about the arrest, Mr Cameron said England and Northern Ireland had an independent judicial system. He added: "We have independent policing authorities, independent prosecuting authorities. Those are vital parts of the free country and the free society we enjoy today."

Meanwhile, Mrs McConville's son Michael McConville spoke of his fears of making a statement to the police in case he or other family members were shot by republican extremists for informing. He said: "The IRA robbed a family of their mother growing old, they took everything away from us," he said.

Mr McConville said he recognised local faces when an IRA gang arrived to drag his mother away screaming in terror from their home in the Divis flats in west Belfast in 1972.

He said: "Everybody thinks that the IRA has gone away but they have not. If we tell we will be shot."

His mother was abducted, shot and then secretly buried - so becoming one of the "Disappeared" victims of the Troubles. Her body was not found until 2003 on a beach in Co Louth.

Mr Adams can be held for 48 hours without charge, with officers having an option to apply to a judge for that detention period to be extended if it is required.

l A 47-year-old man has been arrested on suspicion of dissident republican terrorist activity after explosives were allegedly found at a block of flats in Belfast yesterday.