Michelle Obama -- Can't Stop Malia and Sasha Pics at Beyonce and Jay Z Concert

Michelle Obama was powerless to prevent photos of her daughters from going public -- she and the girls were captured in selfies at Beyonce and Jay Z's concert last night.

The First Lady chaperoned Malia and Sasha to packed Soldier Field in Chicago -- and a concertgoer lined up some of the fam in the background of her selfie ... and then pushed it out to the world on Instagram.

As we first told you ... Michelle called a photo agency earlier this month, and got it to take down shots of Malia walking into a restaurant that's a known paparazzi spot.

The Chi town selfie reveals a huge flaw in the White House's desire to keep the girls out of the public spotlight ... unless they're with the President.

Hard to shut down a stadium full of iPhones.