'Hunger Games' Star Jennifer Lawrence -- 911 Emergency on My Front Lawn

Frightening moment for "Hunger Games" star Jennifer Lawrence yesterday -- when an underage girl collapsed on the front lawn of her L.A. apartment complex.

The Santa Monica Police Department tells TMZ ... Lawrence was out walking her dog ... when she noticed a random girl laid out on her back.

As Jennifer rushed to assist the young woman, someone in the area called 911 ... and paramedics raced to the scene.

Cops tell us ... "The young lady who collapsed was a juvenile and was part of a group that was together."

It's unclear why the young woman was in front of Jennifer's home ... but cops say, "Alcohol may have played a role in the incident."

We're told paramedics didn't feel the girl needed to be transported to a hospital -- instead, she was treated on the scene and cleared to leave.