Justin Bieber -- Ambulance Called To Calabasas Mansion For Passed Out Girl

Ambulances raced to Justin Bieber's Calabasas home early this morning after they received a 911 call saying a woman passed out inside ... TMZ has learned.

We've learned paramedics were dispatched to Bieber's crib this morning around 3 a.m. after someone at Bieber's reported there was a 20-year old unconscious female at the house ... but 15 minutes later the ambulances were called off.

Bieber was home at the time 911 was called. The person who called off the fire department told dispatch the woman would be transported privately to the hospital. But sources familiar with the situation tell us ... the woman regained consciousness and when she was told 911 had been called she insisted they be called off.

We're told the woman did not go to the hospital. She went home instead. We know alcohol was involved. We don't know what, if anything else, was involved.

We're also told the woman was attending a party thrown by Bieber.