Ariana Grande may be fearless -- vowing to return to Manchester after the terrorist attack at her concert -- but she's not foolhardy, because her family home is being guarded by enough security to protect a head of state.

Security vehicles were positioned in front of her mom's home in Boca Raton. It's a gated community, but no one's taking any chances. In addition to private security, 2 cop cars are stationed on the street. 

Law enforcement sources tell TMZ ... Ariana's mom, Joan, always gets extra security when her daughter's in town. However, local police are more actively patrolling the area given the circumstances around Manchester ... and they're calling that shot on their own. 

TMZ broke the story ... Ariana will hold a benefit concert in Manchester a week from today.  All proceeds will go to the family of the victims' families.