Raids on the homes and offices of two journalists were an "outrage" akin to a police state, a court has been told.
The searches in August last year led to the arrest of Trevor Birney and Barry McCaffrey.
Belfast High Court heard the raids were aimed at discovering sources and intimidating whistleblowers.
Mr Birney and Mr McCaffrey are challenging the legality of the search warrants and remain on bail under live police investigation.
Conservative MP and former Brexit secretary David Davis was among those at court supporting the two journalists. He said the case should be highlighted around the world.
The warrants were obtained to carry out the trawls as part of an investigation into the suspected theft of confidential documents from the Northern Ireland Police Ombudsman's office.
The case is connected to the murders of six Catholic men at Loughinisland, Co Down in June 1994.
UVF gunmen opened fire in a village pub as their victims watched a World Cup football match.
The investigative journalists were involved in a documentary film No Stone Unturned, which examined the Royal Ulster Constabulary's handling of the Loughinisland killings.
They were detained, questioned and released during an operation undertaken by detectives from Durham Constabulary, supported by PSNI (Police Service of Northern Ireland) officers.
Judicial review proceedings have been brought in a bid to have the warrants declared unlawful.
The police have given an undertaking not to examine any of the documents and computer equipment pending the outcome of the legal action.
A lawyer for Mr Birney and documentary maker Fine Point Films, told the court the case has "set off alarm bells" among media organisations in Britain, Ireland and the United States who have intervened in support of the journalists.
"This application arises from a police search operation that was nothing less than outrageous," he said.
"Under cover of a warrant that was obtained without giving the applicants the opportunity to be heard, and which should never have been granted, the police raided homes and business premises of two journalists and a film company.
"They rifled through all their confidential files, accessed literally millions of documents, and then seized computers, phones, media storage devices and documentary materials which they still retain - most of them completely unrelated to the pretext on which the search was carried out."
The lawyer continued: "This was the kind of operation associated more with a police state than with a liberal democracy that does have in place laws designed to protect investigative journalists and their sources from this kind of intrusion."
Advancing another possible motive, the lawyer said: "An ulterior motive was to undermine journalists and whistleblowers from exposing misconduct of the police."
The three judges heard police were made aware the film was being made, with the intention of naming suspects whose identities have been in the public domain for more than two decades.
According to the journalists' lawyers, police downloaded millions of documents and pieces of information from their work server.
Mr Birney's lawyer claimed that out of 65 items copied from one computer in the journalists' office, only one was linked to Loughinisland.
He continued: "One set of files related to a case of clerical child sex abuse, that was taken by police knowing it was completely unrelated to Loughinisland."
The case continues.