Justin Bieber -- Pint-sized Trespassers in Pool Party Confrontation

Justin Bieber lost it with 2 young fans, chewing them out and taking one of their cellphones ... but he was more than justified.

We're told a 12-year-old Belieber decided to celebrate her birthday with a friend last Saturday by sneaking onto Justin's estate.

The girls found an opening in the form of an open gate, went inside and came face to face with Justin who was enjoying the 80 degree weather with friends in his pool.

Justin spotted and then berated them for trespassing -- which frankly sounds reasonable -- and then confiscated one of their very active cellphones.

The girls left and cops were not called. We're told they sheepishly returned a few days later and retrieved the phone.

Getting their dignity back ... not so easy.