Raquel Leviss and her family are so terrified after a wave of death threats stemming from her affair with Tom Sandoval, they've contacted the country's top law enforcement source for help ... TMZ has learned.

The "Vanderpump Rules" star's rep tells TMZ ... Raquel and her family have been on the receiving end of violent and graphic death threats both online and over the phone that has gotten so bad they've had to contact the FBI.

We're told some of the alleged threats include messages sent to Raquel through social media, saying ... "Tear you limb from limb" and "Un-alive and dump the body."

A source close to Raquel tells TMZ ... it's understandable why people hate her for the Scandoval, but to elevate that hate to the point of threatening Raquel and her family takes things too far and is disturbing.

A spokesperson for the FBI tells TMZ ... "FBI neither confirms nor denies investigations. Obviously, we review all allegations reported to us and encourage anyone who believes their life is in imminent danger to call 911."

As we reported, Raquel's getting mental health treatment in the wake of the cheating scandal, and we haven't seen her out in public in weeks ... so she's clearly in a bad spot, and the alleged threats aren't helping.

As we reported ... Tom and Raquel were raked over the coals during part one of the "VPR" reunion, with some of their fellow costars threatening Tom with violence.

But death threats are on a whole other level.