Prince Harry has been greeted with cheers and applause as he urged youngsters to help others.
Casually dressed in an open-necked checked shirt, Harry spoke to around 12,000 students and teachers from more than 400 schools at We Day, held at Wembley Arena.
"Some people do not think it is cool to help others - personally I think it is the coolest thing in the world," he told the crowd.
His girlfriend Cressida Bonas was also spotted at the event. It is thought to be the first time she has attended one of his official engagements.
Harry kicked off his speech by joking with the crowd: "For those of you who were expecting Harry Styles here, I apologise and no I am not going to sing."
Students earned tickets for the London event by making a commitment to performing charitable acts locally and globally.
Harry was on stage for 10 minutes but his speech was broken up by cheering, pauses and warm applause.
Admitting to feeling "extremely privileged and incredibly nervous" to be standing on stage, Harry said that children have the power to effect change.
He said: "Every single one of you has done something amazing to be here today.
"It may not feel like it to you, but I can assure you that, collectively, your actions can and will shape the course of our future. After all, we all share this planet, so we must help and inspire others to do the same.
"The world around us is changing fast. On one hand, it is a much more connected place.
"The internet is now part of our everyday lives, and social media has changed the way we communicate.
"What this means is that our generation, although I am nearly 30, has the greatest opportunity of any in our history to effect change on a global scale."
Harry pointed out that we live in a world where there are "very significant challenges" both at home and abroad. Approximately 100 million children each year are affected by disasters such as the Syrian crisis, where one million children have had to flee the country.
Closer to home, many UK youngsters have to deal with domestic abuse, violence and addiction as part of everyday life or provide long-term care for a relative.
These are among many issues which can "rob a child of their childhood. For these children, a little help could go so far".
"And in this way, the biggest impact you can have, as an individual, is within your own community.
"Sometimes the smallest of things, like helping someone cross the road, to climb a flight of stairs or lift their shopping into a car, really does make a difference. It doesn't cost anything, just a little bit of your time.
"Often, without realising it, those we admire most are people who are committed to helping others: to giving their time freely to another, to volunteering like you guys. Every one of you here has inspired others by what you have done, and are doing, so please don't stop."
He hoped that youngsters at risk could be helped by being spotted early, then supported and mentored by someone who has grown up in the same community.
"The mentors I am talking about are not super-human - they are people like you and I who are willing to spend a little time helping someone else," Harry said, urging the crowd to encourage their friends to get involved.
Schoolgirl Malala Yousafzai, who was shot in the head by the Taliban in Pakistan for campaigning for a girl's right to be educated, later told the young crowd: "Your most powerful tool, your most powerful weapon is your voice."
Recalling the day she was shot, she said: "I was shot by the Taliban but on that day the little bit of fear that I had died and courage and strength was born."
US politician Al Gore, actor Clive Owen, entrepreneur Sir Richard Branson and former footballer Gary Neville also made appearances.
Craig Kielburger, who co-founded Free The Children with his brother Marc in 1995, said: "We are absolutely thrilled to bring We Day to the UK for the first time and hope with the help of students across the country we can continue to make positive change locally in the UK and globally.''
We Day events have enabled young people to raise more than £22 million for more than 1,000 causes since 2007.