The ‘Chained To The Rhythm’ singer broke down in tears on stage at the show at the Water Rats venue in the capital.
The US singer dedicated a song to the 22 people who lost their lives after a suicide bomber targeted Ariana Grande fans, following her concert at the Manchester Arena on Monday evening.
Speaking to the crowd, she told them: “This week has been really hard. It sucked. It was hard for you guys and it was hard for me because we all love music.
“We all listen to the same music. And you think about it and think that’s my friend, that’s my sister, that’s my cousin, that’s my brother.
“It’s awful. And whatever you can do to help, you should. And if you can’t do anything that’s fine. But what you should do is not let them win.”
She then dedicated her hit ‘Part Of Me’ to the victims, telling the crowd: “I’m going to sing this next song for them. They can never take that part from us ever. Ever.”
After hearing of the 29-year-old’s death, the US star shared a touching tribute on Instagram.
Posting a picture of Martyn wearing a Mariah T-shirt, she wrote: “Devastated to learn that one of the victims in Manchester was part of the #Lambily. RIP Martyn Hett. We will cherish your memory forever.
“His family and all the families affected are in my thoughts and prayers. MC.”
Radio 1 Breakfast Show host Nick Grimshaw previously shared some poignant words on his social media accounts about Martyn.