The Duke of Sussex was on hand, in the flesh, Tuesday ... where he hit the witness box early and reportedly read from a witness statement he'd prepared to discuss the allegations he's levied against the Daily Mirror -- the first tabloid case of his being heard.

His statement was reportedly 55 pages long, touching on everything from former associates/employees of Princess Diana -- including her ex-butler, Paul Burrell, whom he called a "two-faced s***" -- to voice mails of his that he claimed were accessed before he could ever hear them.

There's one compelling line about how he felt during the mid-2000s through the early 2010s ... the period of time at issue in the alleged phone hacking/spying case. Harry said, "Despite the common misconception, I was no more than 5% funded by the taxpayer while I was a working Royal in the UK, yet it felt as though the tabloid press thought that they owned me absolutely, and deserved to know everything there is to know about me, my life, my movements and the lives of those people who came into my orbit."

As for why he felt his phone was being hacked ... he described getting notifications for new voice mails on his phone back then -- but then seeing the notification disappear before he could even hear the audio. He says this happened often.

There's more ... Harry said stories that ran during this time about him possibly having been fathered by James Hewitt were incredibly hurtful to him, and harmed his mental health -- sparking fears he'd be ousted from the Royal Family amid speculation.

And lastly, he took direct shots at Piers Morgan ... whom he alleged was the one signing off on the alleged spying/hacking, as the guy was top dog at Daily Mirror at the time. He also alleges Piers is behind recent Mirror attacks against him and his wife, Meghan Markle.

One final telling quote from Harry ... "How much more blood will stain their typing fingers before someone can put a stop to this madness?" When cross-examined on that remark, PH responded ... "Some of the editors and journalists that are responsible for causing a lot of pain, upset and in some cases, speaking personally, death."

There was further cross-examination, in which attorneys for the Mirror asserted the articles he's suing over had actually already been written by other outlets -- and that the Mirror was simply riding the news cycle at the time with their own versions.