Kate Middleton and daughter Princess Charlotte excitedly clapped for the Queen at tonight's Platinum Jubilee concert.
The royals are sitting two rows ahead of Prime Minister Boris Johnson, who was booed by crowds outside St Paul's Cathedral yesterday, and his wife Carrie.
The Queen herself is understood to be watching the show on TV, having been forced to miss events today because of mobility problems.
Rock band Queen and Adam Lambert go the show underway, as large crowds gathered outside Buckingham Palace to soak up the atmosphere.
The concert will feature A-list stars including Alicia Keys, Diana Ross and Nile Rodgers.
Prince William and Prince Charles will both deliver speeches praising the 96-year-old monarch, who has been on the throne for 70 years.
Earlier today William and Kate were in Wales to check in on rehearsals at Cardiff Castle for a celebration event there tonight.
They brought their youngest children, but youngest son Louis - who won hearts when he covered his ears during Thursday's flypast over Buckingham Palace - was absent.
Kate and Charlotte were both filmed conducting musicians as they launched into a stirring version of Sweet Caroline earlier today.
After dashing back to London for tonight's festivities, William and his eldest son, Prince George, were seen talking, with the youngster beaming as the show got underway.
They joined other senior royals, but William's brother Harry and wife Meghan Markle were not with them.
Comedian Lee Mack made a joke about partygate in front of Boris Johnson as he addressed the crowd.
Speaking in front of the Prime Minister, who was in the royal box, he said: "Finally we can say the words 'party' and 'gate' and it's a positive."
The joke drew cheers and laughs from the crowd.
Mack then replicated former Queen frontman Freddie Mercury's "Ay-Oh" chant with the crowd, prompting laughs from the Duke of Cambridge and his children.
Lord Andrew Lloyd Webber has said it is "almost impossible to put into words" everything the Queen has done for the UK.
The impresario has put together a selection of musical theatre shows featuring stars including Lin-Manuel Miranda for the BBC's Platinum Party at the Palace concert.
Speaking backstage, Lord Lloyd-Webber told the PA news agency: "The thing about the Queen is that she is most extraordinary in her sense of duty, everything that she has done for our nation, and to try and harmonise everybody and bring everybody together is almost impossible to put into words.
"I think maybe young people don't understand the sense of duty which she has, which is just really utterly extraordinary.
"And she will never, ever do anything where she puts herself before duty."
He added that he feels the best thing about the monarch is that "she never actually says what she thinks" as she is always complimentary, so he tries to "read through whether it's something that she really enjoys".
