• Prince Harry made a surprise appearance during a virtual ceremony for the Diana Award 
  • He said he sees his late mother Princess Diana's legacy in the awardees and his children Lilibet and Archie
  • Prince William also honored their late mother in his letter to the Diana Award recipients

Prince Harry and Prince William honored their mother Princess Diana on what would have been the late Princess of Wales' 61st birthday.

On Friday, the Duke of Sussex made a surprise appearance during the virtual ceremony for this year's Diana Award, a prestigious award named after the late royal that honors people between the ages of 9 and 25 for their social action and humanitarian work.

In a video address, Prince Harry thanked the young philanthropists for helping "keep her voice alive" 25 years after her tragic 1997 death.

"There isn't a day during the past two and half decades where I haven't thought about the mark she left not only on me and my brother [Prince William], but on all of our lives," Prince Harry was quoted as saying by People.

"I see her legacy in all of you," he continued. "I see her legacy in a Diana Award community that spans multiple generations. I see her legacy every time I meet with families, young people, and children from all corners of the world. And, I see my mum's legacy when I look at my own children every day."

Prince Harry went on to share that Princess Diana taught him to strive for a "better world" and that his mother's voice became "stronger" in his life after he became a husband and a father.

Prince Harry shares son Archie, 3, and daughter Lilibet, 1, with wife Meghan Markle.

"I know the memory I hold of my mum will never fade, because I see her spirit in each of you. So, thank you for keeping her fire alive," he told the award recipients in a final appearance during the ceremony.

Prince William also honored their late mother's legacy by sending a letter to the Diana Award winners.

The second in line to the British throne praised the young philanthropists for "changing the world" through their actions and thanked them for their "compassion, bravery and absolute determination."

"You truly are the personification of my mother's legacy and I know she would be so proud of you all," he wrote, E! News reported. "There's no better way to celebrate her life and work than through recognising incredible people who dedicate so much time and effort to helping those around them."

Friday's award ceremony honored 180 changemakers and young philanthropists from 29 countries — including more than 20 winners from the U.S.

Among the winners was 17-year-old Priya Patel, from New Jersey, who founded Peer Responders. Prince Harry described it as "a groundbreaking organization with a mission to combat depression and suicide through friendship."

Prince William and Prince Harry provided the enduring image from Diana's funeral, as they stood, head bowed, in grief
Prince William and Prince Harry provided the enduring image from Diana's funeral, as they stood, head bowed, in grief POOL / ADAM BUTLER