Meghan and Harry's tour ends as Mail on Sunday vows to defend itself in court
The Duke of Sussex has told an event in Johannesburg that he and his wife will "seek to challenge injustice".
His comments come a day after it emerged that they were taking legal action against the Mail on Sunday for publishing a private letter sent by the Duchess of Sussex to her father.
The duke said the legal action was in response to "relentless propaganda".
The paper says it will defend itself vigorously and stood by the story it published.
On the final day of their 10-day overseas tour, Prince Harry set out what he believes his role in public life should be, saying he and the duchess would "stand up for what we believe".
Speaking to a group of young people and fledgling entrepreneurs in Tembisa township, near Johannesburg, the duke said: "We are fortunate enough to have a position that gives us amazing opportunities and we will do everything that we can to play our part in building a better world.
"We will also seek to challenge injustice and to speak out for those who may feel unheard.
"So no matter your background, your nationality, your age or gender, your sexuality, your physical ability, no matter your circumstance, or colour of your skin - we believe in you.
"And we intend to spend our entire lives making sure that you have the opportunity to succeed and change the world."
Prince Harry went on to reminisce about a visit to Africa in the months following the sudden death of his mother Diana, Princess of Wales.
"Ever since I came to this country as a young boy, trying to cope with something I could never possibly describe, Africa has held me in an embrace that I will never forget and feel incredibly fortunate for that," he said.
"Every time I come here I know that I'm not alone. I always feel wherever I am on this continent that the community around me provides a life that is enriching and is rooted in the simplest things - connection, connection with others and the natural environment."
Prince Harry said he wanted to teach his baby son Archie the lessons he had learned from Africa, including those about "community and friendship".
Later, in a speech at the Johannesburg residence of Britain's high commissioner, the duchess said people have the power to change a world that seems "aggressive, confrontational and dangerous".
Meghan told designers, entrepreneurs and business people: "Whether you're here in South Africa, at home in the UK or the US, or around the world, you actually have the power within you to change things, and that begins with how you connect to others."
Later in the day, the duke and duchess met Nelson Mandela's widow, Graca Machel. She offered to work with the couple, who launch their Sussex Royal Foundation next year.
The law firm Schillings, acting for the duchess, has filed a High Court claim against the Mail on Sunday and its parent company - Associated Newspapers - over the alleged misuse of private information, infringement of copyright and breach of the Data Protection Act 2018.
The duchess's action comes after the newspaper published a handwritten letter she sent her father shortly after she and Prince Harry got married in 2018.
The paper is accused of an "intrusive and unlawful publication of a private letter" and of a campaign of publishing false and derogatory stories about the Duchess of Sussex.
Sometimes there are exceptions to copyright which can allow part of a letter or document to be published, for example for reporting current events.
But even if this is used, under what is known as the "fair dealing" defence, publications have to strike a balance between public interest and the interest of the copyright owner.
Referring to his late mother Diana, Princess of Wales, Prince Harry said his "deepest fear is history repeating itself".
In a lengthy personal statement on the couple's official website, he said the "painful" impact of intrusive media coverage had driven him and his wife to take action.
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Prince Harry said: "I lost my mother, and now I watch my wife falling victim to the same powerful forces.
"I've seen what happens when someone I love is commoditised to the point that they are no longer treated or seen as a real person," he added.
Diana was once described as the "most hunted person of the modern age".
She died in a car crash in 1997 after being pursued through Paris by a pack of paparazzi journalists.
The new legal proceedings are being funded privately by the couple and any proceeds will be donated to an anti-bullying charity.
In his statement Prince Harry said he and Meghan believed in "media freedom and objective, truthful reporting" as a "cornerstone of democracy".
But he said his wife had become "one of the latest victims of a British tabloid press that wages campaigns against individuals with no thought to the consequences".
The duke accused the paper of misleading readers when it published the private letter, by strategically omitting paragraphs, sentences and specific words "to mask the lies they had perpetrated for over a year".
"Put simply, it is bullying, which scares and silences people. We all know this isn't acceptable, at any level," he said.
The Mail on Sunday spokesperson said: "We categorically deny that the duchess's letter was edited in any way that changed its meaning."