Stars arrived at Alexandra Palace on Sunday for the inaugural Earthshot Prize ceremony, where five £1 million grants were awarded for environmental projects.
The scheme was launched by Prince William, with the backing of a council including Sir David Attenborough, and will run for at least five years, with next year's event hosted in America.
The evening was co-hosted by Dermot O'Leary and Clara Amfo, and prizes went to projects in Costa Rica, Italy, India and the Bahamas, and to a firm based across Thailand, Germany and Italy.
The Duke of Cambridge has issued a rallying cry to the next generation to keep on “demanding change”. He called on the younger members of society not to “give up hope” during the event.
At the end of the event, televised by the BBC, William, who was joined by Kate, took to the stage to tell the audience: “I want to say something to the young watching tonight. For too long, we haven’t done enough to protect the planet for your future, the Earthshot is for you.
"In the next 10 years, we are going to act. We are going to find the solutions to repair our planet.
“Please keep learning, keep demanding change, and don’t give up hope. We will rise to these challenges.”
The Republic of Costa Rica, whose policy of paying citizens to protect the rainforests and restore ecosystems reversed decades of deforestation, will expand its work to protect the ocean and support the replication of its approach elsewhere.
New Delhi-based Takachar has developed technology to help end the burning of agricultural waste, and Coral Vita, in the Bahamas, has created a way of growing coral on land and replanting it in the ocean, enabling it to grow up to 50 times faster than traditional methods.
The City of Milan's hubs to redistribute food waste will be scaled up to other cities, while the AEM Electrolyser turns renewable electricity into emission-free hydrogen gas.
A pre-recorded performance by Coldplay of their new single My Universe opened the event, powered by 60 people pedalling stationary bicycles in the park.
Actor Emma Watson wore a wedding dress made of 10 dresses from Oxfam, with guests asked not to buy new outfits for the ceremony.
"Can you imagine the relief?" said Emma Thompson, who re-wore her "Dame suit", complete with vegan sneakers.
"It's very hard contemplating the fact of climate change, particularly the facts of waste," she said before the event.
"But one of the great things about this prize is that it addresses one of the things that I think worries us very deep in our consciousness, which is this sense of waste – wasted food. You go to a supermarket and what's going to happen to all this food at the end of the day? Where's it going to go?
"That sense of hopelessness in the face of this huge problem... one of the solutions is going to be here, and that makes me very happy, and that's why I think it's so important."
Prince William said: “Our five inspirational winners show that everyone has a role to play in the global effort to repair our planet. We need businesses, leaders, innovators, and communities to take action.
"And, ultimately, we need all of us to demand that the solutions get the support they need. Because the success of our winners is our collective, global Earthshot.”
Sir David Attenborough said: “The natural world on which we entirely depend is declining at a rate faster than at any time since the end of the dinosaurs.
"We know where this story is heading and we must now write a different ending. This is what the Earthshot Prize was created to achieve.
"The 15 Earthshot Prize finalists tonight build optimism by finding innovative and brilliant solutions to the world’s challenges, and they give us hope, which, we are told, springs eternal.”
The winners were selected by the Earthshot Council, consisting of Prince William, Queen Rania of Jordan, Sir David Attenborough, Dr Ngozi Okonjo-Iweala, Indra Nooyi, Shakira, Christiana Figueres, Luisa Neubauer, Cate Blanchett, Yao Ming, Dani Alves, Ernest Gibson, Hindou Oumarou Ibrahim, Jack Ma, and Naoko Yamazaki.
The ceremony capped a 10-month global search with more than 750 applications from around the world. 15 finalists were chosen through a selection process, supported by an expert advisory panel, for their potential to positively impact people and the natural world and their ability to reach Earthshot's goals.
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