Cincinnati Zoo: 911 Call ... 'My Son Fell In With a Gorilla!'

Terror erupted moments after that 3-year-old boy fell in Harambe the gorilla's enclosure at the Cincinnati Zoo ... and we just got the 911 call.

In the emergency call -- released by cops -- you can hear Michelle Gregg's panicked conversation with dispatch ... screaming that her son had just fallen into the gorilla pen.

Michelle tells her son to "be calm," as she exclaims, "He's dragging my son. I can't watch this."

As we previously reported ... cops are reviewing the incident to see if any fault lies with the boy's parents.