The X Factor UK judge was on the main stage at the Hyde Park event when she dedicated a cover of David Guetta's 'Titanium' to the man at the centre of her sex tape scandal earlier this year.
Addressing the crowd, Contostavlos said: "Put your middle fingers up to my d**khead ex, Justin f**king Edwards.
"Ladies, don't ever let any man pull you down, because I'm still f**king here."
A source told The Sun: "Tulisa said it because she felt the lyrics to 'Titanium' were very apt for what he did to her.
"It was a spur-of-the-moment decision. The crowd were totally behind her, all sticking up the middle finger to him."
Copyright: Ian West/PA Wire/Press Association ImagesAnother audience member told EntertainmentWise that Contostavlos's words were well received, despite a mixed performance from the 'Young' singer.
"She was awkward in the sense that nobody knew her songs or could hear her as her mic levels were so low," the spectator said.
"But the ex-boyfriend bit, I think people just cheered and got on with it."
Last week, a user claiming the N-Dubz star was a manufactured pop act was bombarded with abuse from the 23-year-old's followers.
Watch the video for Tulisa Contostavlos's single 'Young' below: