Scotland's police complaints watchdog has been accused of using "oppressive and dehumanising" tactics as part of its investigations.
The Scottish Police Federation (SPF), the body which represents rank and file officers, also complained that its members are "fast losing confidence in the effectiveness and genuine independence" of the Police Investigations and Review Commissioner (Pirc).
In a submission to MSPs on Holyrood's Justice Sub-Committee on Policing the SPF said that officers "may be told they are witnesses but they are treated like suspects" in Pirc's investigations.
Pirc is currently examining the circumstances of the death in police custody of Sheku Bayoh last year, and Police Scotland's failure to respond to reports of a crash on the M9 motorway which killed John Yuill and his partner Lamara Bell.
SPF general secretary Calum Steele said: "Our members have reported that during Pirc investigations, they may be told they are witnesses but are treated like suspects. This inevitably makes it difficult for our members to assess whether they are in fact being treated fairly or whether they may be potential suspects in future proceedings.
"Our members have reported examples of being 'interviewed' for hours on end without rest and one apparent witness reported that they were only able to use a toilet, during a seven hour interview / interrogation provided they were accompanied by a Pirc investigator.
"Quite simply these type of oppressive and dehumanising activities risk fatally undermining the Pirc and should have no place in any fair investigatory process."
A Pirc spokeswoman said: "The commissioner has only recently been made aware of the content of the Scottish Police Federation letter to the Justice Sub-Committee and has requested the federation to provide specific information relating to the concerns raised so that she may address those matters and take any appropriate action.
"We look forward to receiving the information requested in due course."
The SPF also raised concerns that officers are "over-zealously" pursued by Police Scotland for "minor matters" such as being drunk while off-duty or flirting.
Mr Steele said: "The manner in which the Police Service of Scotland overreacts and seeks to punish officers for minor matters leads to deep-rooted mistrust and presumption against openness on minor misconduct for fear of overzealous consequences."
Officers risk misconduct investigations if they have "vomited on a night-out, were asked to leave licensed premises as a result of being ill, consumed excess alcohol whilst off-duty, had the temerity to send a text message asking a colleague on a date or complimenting her/him on their appearance", he said.
Police Scotland said it would be inappropriate to comment ahead of Thursday's committee meeting.