Demi Lovato kicked off her birthday celebrations with Ariana Grande.

The pop star turned 27 on Tuesday but started the party a day early by attending Grande’s concert at London’s O2 Arena on Monday evening.

Before Grande walked on stage, she was joined by her backing dancers and manager Scooter Braun in cheering “happy birthday, Demi”.

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Last day as 26 🖤

A post shared by Demi Lovato (@ddlovato) on

The two pop stars then shared an embrace.

Lovato shared a video of the moment with her 73 million Instagram followers.

She captioned the post: “This was too sweet not to post…. before show prayer they did this for my bday. I’m so so proud of you @arianagrande.

“You f****** killed that!!! I love you tons.” Grande, 26, commented on the post: “Love you so much!! Thank u for coming.”

Lovato, who had earlier shared a selfie captioned “last day as 26”, also posted a video of herself blowing out candles on a birthday cake while Grande and her friends clapped and cheered.

Alongside the clip, Lovato wrote: “So so happy. And I love my new family. Thank you guys, love you.”

Lovato was a child star and found fame on the TV series Barney & Friends before appearing in the Disney Channel film Camp Rock.

She has since gone on to be a multi-platinum pop star with hits such as Sorry Not Sorry, Skyscraper and Heart Attack.

Lovato has been open about her struggles with alcohol and substance abuse and in July last year was taken to hospital after suffering a suspected overdose.