Madonna surprised 400 fans at an anniversary screening of 'Madonna: Truth or Dare'

A source told PEOPLE magazine: "She came out to support Alek Keshishian, her longtime friend and director of the celebrated documentary."

Meanwhile, the 58-year-old singer - who has Lourdes, 19, Rocco, 16, Mercy and David, both 10 - previously admitted she thinks it's "easy to become famous".

She said: "I was already famous before social media, so for me fame isn't the burden. Fame is the manifestation or the by-product of my work, and that was two decades before social media. Now to me the burden is people are more focused on fame than actually doing the work or being an artist. Now it's easy to become famous. What isn't easy is to develop and grow as an artist without being distracted or consumed with fame.

"I don't consider myself a pop act, I consider myself an artist. And it's an artist's responsibility to be revolutionary in our work. It's our responsibility, our duty and our privilege."

And Madonna also recently revealed she is a big fan of Instagram and sees it like "keeping a diary".

She said: "I like Instagram because it's like keeping a diary and every day I get to share different aspects of my personality, my life, and what inspires me, what infuriates me, or what causes I want to fight for.

"It allows me to be mysterious, ironic, provocative or proud. I get to use it as a platform to bring attention to people or issues that I think are important. It allows me to be the curator of my life."