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Prince William, Duchess Kate and Prince Harry in high spirits as they take a ride on the London Eye

Gemma Strong

The London Eye hosted some very special guests on Monday as Prince William, the Duchess of Cambridge and Prince Harry became the first members of the British royal family to ride the iconic landmark. The trio were clearly in high spirits as they arrived on London's Southbank, laughing and chatting as they boarded one of the pods together.

Huge crowds had gathered along the Thames to catch a glimpse of William, Kate and Harry, and as they began their slow ascent on the giant Ferris wheel, they smiled and waved down to spectators gathered below.

Heads Together World Mental Health Day 2016 from InVision Communications on Vimeo.

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The Duchess of Cambridge arrives at the London Eye

William, Kate and Harry stepped out in London to show their support for their Heads Together initiative on World Mental Health Day, and were joined for the ride by some of those who had bravely shared their mental health experiences with them at a reception at London's County Hall.

The trio spearheaded the Heads Together campaign last May to try and end stigma surrounding mental health – it is the single biggest project they have undertaken all together, and is clearly an issue close to their hearts.

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Prince William and Prince Harry share a joke as they prepare to board a capsule

On Monday evening, the London Eye will light up in purple in support of World Mental Health Day, joining other buildings around the world that will be doing the same, and connecting Heads Together with the worldwide movement to end stigma around mental health.

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The global Light Up Purple drive was set up in memory of Canadian teenager Amanda Todd, who tragically killed herself in 2012. The 15-year-old brought the problem of cyber bullying to mainstream attention after she posted a video on YouTube in which she told her story with handwritten signs.