Police were called to Britney's Beverly Hills home at 8pm after the 25-year-old star refused to return her sons, two-year-old Sean Preston and one-year-old Jayden James, to ex-husband Kevin Federline.
Officers found Britney to be "under the influence of an unknown substance", and she was taken to Cedars-Sinai Medical Center in an ambulance where she is reportedly undergoing a psychiatric evaluation.
Los Angeles Police Department officer Jason Lee said: "While officers were on the scene, they observed Ms. Spears under the influence of an unknown substance. She is now being taken in for an evaluation."
Several police officers, fire engines, two ambulances and a police helicopter all appeared at the tumultuous scene.
When Britney arrived at the medical center, she stuck two fingers up at waiting paparazzi but smiled as she was wheeled out of the ambulance.
One onlooker said: "Britney looked completely out of it, but still managed to flip off the photographers and smiled at the same time. She is just unbelievable."