The British royal and other claimants accuse Associated Newspapers of unlawful behaviour from 1993 until 2011 and beyond in one of the most high-profile civil cases in the UK for years.
Harry, 41, who arrived smiling and waving at court, said in a witness submission it was "disturbing to feel that my every move, thought or feeling was being tracked and monitored just for the Mail to make money out of it".
Associated calls the allegations "preposterous smears" and part of a co-ordinated conspiracy by a wealthy group driven by personal animosity towards the media.
Over nine weeks, the legal team for Harry, singer John and the other claimants - John's husband David Furnish, actors Liz Hurley and Sadie Frost, anti-racism campaigner Doreen Lawrence and former lawmaker Simon Hughes - will argue private investigators instructed by the Mail unlawfully obtained material about them.
Their lawyer David Sherborne said practices included hacking voicemail messages, bugging landlines and obtaining private information by deception, known as "blagging".
"There can be little doubt that journalists and executives across the Mail titles engaged in or were complicit in the culture of unlawful information gathering that wrecked the lives of so many," Sherborne said in documents released at the start of the trial at London's High Court on Monday.
He said the company's vigorous denials, destruction of records and "masses upon masses of missing documents" had prevented the claimants from learning what the newspapers had done.
"They swore that they were a clean ship," Sherborne said. "Associated knew that these emphatic denials were not true. … They knew they had skeletons in their closet."
Harry, 41, who has long blamed the press for the death of his mother in a Paris car crash in 1997 as her vehicle sped away from paparazzi, listened on in court, sitting behind his lawyers with Frost and Hughes.
Among those accused of involvement in wrongdoing were managing editors and senior journalists including the current editors of the Mail on Sunday and Sun newspapers.
Examples included finding out precise travel plans of Harry's former girlfriend Chelsy Davy, and a report about "private and intimate conversations" between Harry and his elder brother Prince William about a statement regarding images of their dead mother, the claimants' written submissions said.
In his witness statement, quoted in the submissions, Harry, whose case is based on 14 published newspaper stories, says the intrusion had been "terrifying" for loved ones, and created a "massive strain" on personal relationships.
Associated, whose titles had not previously been embroiled in the phone hacking scandal hanging over the British press for two decades until the case was filed in 2022, has denied wrongdoing and says the lawsuits should have been brought sooner.
It says evidence from former private detectives - some of whom have been convicted of crimes and whom Associated say have been paid by the claimants' legal team for their testimonies - was untrustworthy.
The publisher says the claimants, backed by rich supporters, were trying to reopen a public inquiry into press standards which was held almost 15 years ago after revelations of phone hacking caused public outrage.
"The allegation that these practices were 'habitual and widespread' at Associated's titles was simply untrue," the publisher said in its written submission.
For Harry, it is the final instalment of his legal war on the British tabloids, having said it was his mission to clear up the press and hold those in senior positions to account.
He has already successfully sued Mirror Groups Newspapers (MGN) for damages while he won an apology and admission of some wrongdoing by Rupert Murdoch's British newspaper arm which settled ahead of a trial a year ago.
The prince is due to give evidence on Thursday, having become the first British royal to appear in a witness box in 130 years in 2023 during the MGN trial.
With AP
