The Duke of Sussex, 41, and six other claimants including singer Elton John are suing the Mail's publisher Associated Newspapers for violations of their privacy from the early 1990s until the 2010s.
Associated has called the allegations "preposterous smears", saying their journalists had legitimate sources for information, including from the celebrities' friends and acquaintances.
Having become in 2023 the first royal in 130 years to give evidence in court during another of his lawsuits against the press, the younger son of King Charles appeared as the claimants' initial witness in the case against Associated.
The prince's case centres on 14 articles his legal team says were the product of unlawful information gathering, including by hacking voicemail messages, bugging landlines and obtaining private information by deception, known as "blagging".
Associated, however, says Harry's social circle was known to be "leaky" and that some articles were based on public statements issued by royal press officers.
The publisher's lawyer, Antony White, put it to Harry that a former royal editor of the Mail on Sunday, Katie Nicholl, was part of his social circle.
"For the avoidance of doubt, I am not friends with any of these journalists and I never have been," Harry said in combative exchanges with the lawyer.
"If the sources were so good and she was hanging out with all my friends, then why was she using private investigators who have been connected to all the unlawful information gathering?"
Harry and the other claimants launched their legal action against the publisher of the influential Daily Mail in 2022, for the first time dragging Associated's titles into a phone-hacking scandal that had long dogged the British press.
The others involved are Elton John's husband David Furnish, actors Liz Hurley and Sadie Frost, anti-racism campaigner Doreen Lawrence and former MP Simon Hughes, who will all give evidence during the nine-week trial.
Their lawyer David Sherborne said on Monday there was "clear, systematic and sustained use of unlawful information gathering" at the Daily Mail and the Mail on Sunday.
Associated's lawyers, however, say its current and former journalists will come to court to give evidence and present "a compelling account of a pattern of legitimate sourcing".
For Harry, who has long blamed the press for the death of his mother Princess Diana in a Paris car crash in 1997 as her vehicle sped away from paparazzi, the trial is the final instalment of his legal war on the British tabloids.
He has already successfully sued Mirror Group Newspapers, when he also gave evidence in person, and won an apology and admission of some wrongdoing by Rupert Murdoch's British newspaper arm, which settled ahead of a trial a year ago.
In his witness statement, Harry said his battle with the press was a "public duty" and one of his most important roles.
Harry was originally due to give evidence on Thursday but this was brought forward after Associated concluded their opening argument much faster than expected.
