Demi Lovato celebrated her 27th birthday at Ariana Grande's concert in London on Monday (19.08.19).

While Demi's birthday is on Tuesday (20.08.19), the star started her celebrations early with manager Scooter Braun and his client Ariana at London's 02 arena, where Ariana was performing on her 'Sweetener' world tour.

Demi first shared a selfie with the caption: ''Last day as 26'' and then posted a video from the pre-show prayer circle, where she and Ariana shared a sweet hug.

Demi wrote: ''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**king killed that!!! I love you tons (sic).''

Ariana responded: ''love u so much !!! thank u for coming!!!! (sic).''

And Demi, Scooter and Ariana also enjoyed some birthday cake. Demi shared a video of her being presented with the cake and wrote: ''Okay, last one before bed. They literally filmed/sang happy birthday twice and @scooterbraun STILL forgot to press record. @arianagrande's face is everything and this video is a perfect glimpse of how f**king happy and rad my birthday is this year. So so happy. And I love my new family. Thank you guys, love you (sic).''

During the concert, Demi uploaded a number of videos to her Instagram stories as she hung out with Max Martin and Scooter.

Back in May it was revealed that Demi - who suffered a drug overdose in July last year - had hired Scooter to be her manager.

She wrote on Instagram: ''GUYS!!!!!!! Dreams came true today for me. I officially have a NEW MANAGER!!! And not just any new manager but the one and only @scooterbraun!!!! Couldn't be happier, inspired and excited to begin this next chapter with you Scooter!!! Thank you for believing in me and for being apart of this new journey. Let's DO THIS!!!!!! (sic)''

Previously, Demi opened up about feeling ''sober and grateful to be alive'' after her overdose.

The pop star - who spent time in rehab after she was found unconscious at her Los Angeles home - reassured fans she was fine but also asked for some space.

She wrote on Twitter: ''I am sober and grateful to be alive and taking care of ME...

''I'm so blessed I get to take this time to be with family, relax, work on my mind, body and soul and come back when I'm ready. I have my fans to thank for that. I'm so grateful, truly. I love you guys so f**king much thank you (sic).''