Harry, the younger son of King Charles, has brought a lawsuit against Associated Newspapers (ANL), along with John, and his husband David Furnish, as well as actors Elizabeth Hurley and Sadie Frost.
They allege they were victims of "numerous unlawful acts" carried out by journalists or private investigators working on behalf of ANL titles the Daily Mail and the Mail on Sunday.
These included hacking mobile phone messages, bugging calls, getting private information such as medical records by deception or "blagging", and "commissioning the breaking and entry into private property", their lawyer David Sherborne said. The alleged activity ran from 1993 to 2011, and even up to 2018.
ANL, which is seeking to have the case thrown out, said in a statement it categorically denied the allegations and would vigorously defend them if necessary.
Harry, who flew in from his California home, sat just feet away from reporters, watching intently and taking notes. His spokesperson said he wanted to be there to show his support.
Elton John, Furnish, Frost and another claimant, Doreen Lawrence, mother of Black teenager Stephen Lawrence who was murdered in a 1993 racist attack, were also in court on Monday.
None of the claimants, who also included former lawmaker Simon Hughes, are expected to speak during the four-day preliminary hearing, which they were not required to attend.
Harry hugged Lawrence at the end of the day's hearing and chatted to her and Furnish, and gave a thumbs up to crowds outside as he left court.
He is not expected to see his father King Charles or elder brother William while he is in London, while his attendance at the new monarch's coronation in May is still not confirmed.
In his claim, Harry accuses the papers of using unlawful means to obtain stories about him from at least early 2001 until at least late 2013, and said his brother William and the mother of William's wife Kate had also been targeted.
It alleges they had sought information about private flight details of his ex-girlfriend Chelsy Davy, and hacked mobile phone voicemail messages and bugged the landlines of his friends, with a hardwire tap put on the phone on one of them, Guy Pelly.
The actions had meant he was "largely deprived of important aspects of his teenage years" and the suspicion and paranoia they caused led to him losing friends as "everyone became a 'suspect'", the claim said.
The unlawful attempts to find out details of the royal's private travel plans were also a significant security risk "which was as grossly irresponsible as it was dangerous", the document said.
In court submissions, ANL said the claims were based on inference rather than evidence, and that the claimants had provided little or no evidence of unlawful information gathering by its journalists – which it strongly denies.
The publisher's lawyers are arguing that the claims fall outside a time limit for legal action and that some breach an order made during a year-long public inquiry into press standards which began in 2011.
That inquiry, headed by senior judge Brian Leveson, followed outrage over reporters hacking voicemail messages. The scandal led to the closure of Rupert Murdoch's News of the World tabloid in 2011 and later the jailing of its former editor.
Media intrusion was one of the reasons Harry and Meghan, the Duke and Duchess of Sussex, cited for stepping back from royal duties and moving to California to forge new lives and careers.
They attacked the press in their recent six-part Netflix documentary series and in Harry's memoir "Spare", and the latest case is one of a number of ongoing lawsuits in which he is involved.