The Law Society has demanded a meeting with Chief Constable Sir Hugh Orde over the arrest of a Limavady solicitor.
Mr Sandhu is a solicitor based in Limavady
Manmohan Sandhu, 41, is being questioned about serious crime. Two other men have now been released.
His solicitor, Joe Rice, alleges police gathered evidence by covertly taping confidential conversations between Mr Sandhu and his clients.
Law Society chief executive John Bailie said it was an "intrusion into the solicitor-client relationship".
"A client in these circumstances - who is arrested and in police custody, suspected of having committed a criminal offence - needs to be able to speak to his solicitor candidly," he said.
"The administration of justice requires that should happen: he needs to be guaranteed that it is kept confidential.
"I think also the reason why we are taking this so seriously is that it is the first occasion I am aware of in Northern Ireland that there has been this kind of intrusion into the solicitor-client relationship."
Mr Rice wrote to the Law Society, the body representing the legal profession, to complain about how, he claims, the police gathered evidence in the case.
Mr Rice alleges the conversations, allegedly taped at Antrim police station, led to Mr Sandhu being questioned about serious terrorist activity including membership of a loyalist paramilitary organisation.
The letter from Mr Rice to the Law Society's chief executive also said: "It is a sad day for our criminal justice process that a solicitor cannot guarantee that his advices to his client in a police station may not be free from state interference."
He also states: "I am sure you will share my concern that the right to confidentiality that must exist between solicitor and client has now been eradicated.
"This is a deliberate move by the authorities and no solicitor can at present guarantee his client that any pre-interview or indeed post-interview consultations at police stations in Northern Ireland are private and confidential."
Police refused to confirm or deny the identity of Mr Sandhu on Thursday, but confirmed they have been granted another 48 hours to question a man at Antrim police station.
A police spokeswoman said on Thursday that two of three men arrested about serious crime on Tuesday had been released, pending reports.