Britney Spears’ free concert in Mexico City over the weekend caused approximately 80,000 pop fans to flock to the Monumento a la Revolución to see the pop star’s Femme Fatale concert in action. Just how many is 80,000? As this pic, taken from the stage and uploaded by Brit — it’s basically a claustrophobic’s worst nightmare. (Head here for a larger view.) “Thx to the 80,000 fans that showed up in Mexico City,” tweeted Spears. “Can’t imagine what you’d do if u had to go to the bathroom!” Us, neither.