The group at Spears' house included plainclothes officers, motorcycle police, ambulance crews and some police "brass," the official said.
The plan to transport Spears had been in the works for the better part of the day, the official said.
The Los Angeles Times reported that the pop star was taken to UCLA Medical Center.
Andrew Blankstein, a Los Angeles Times reporter who was at the scene, told CNN that a police convoy, stretching the length of a football field, accompanied the ambulance as it drove down Coldwater Canyon Boulevard -- in part to keep the paparazzi from getting too close.
Spears was whisked out a side entrance to her house.
"You couldn't really see what was going on [when officers first swept into the residence] but on the police radio, there was some reference to 'The package is on the way out,' 'We're leaving now. Go, Go, Go,'" Blankstein said.