While Justin and Hailey were partying at Coachella, we snapped these exclusive pix of an intruder being arrested at the couple's new Beverly Hills mansion!

Mr. and Mrs. Bieber attended the Bootsy Bellows Pool Party Presented by McDonald’s & PacSun, taking in a night-time concert with friends like Gigi Hadid, Kendall Jenner, Jaden Smith, and Wiz Khalifa.

But back to Beverly Hills ... around noon on Sunday, we spotted a young woman snooping around outside Bieber's house and immediately after our photographer spotted her, police rolled up and arrested her. She was wearing pajama-like clothes and appeared to have two hospital bracelets around her left wrist. She was calm and went willingly with police. Yikes!