A Los Angeles police officer, who requested anonymity because he was not authorized to speak on the matter, said the 26-year-old pop star was being taken to "get help." The Los Angeles Times cited unnamed authorities who said Spears was being placed on a "mental evaluation hold."
Spears was taken to UCLA Medical Center, where her mother, Lynne, was seen leaving its psychiatric hospital at about 5:30 a.m.
When asked by a throng of paparazzi and reporters whether Britney was doing all right, Lynne replied "Yeah," before leaving in a Range Rover.
Hours earlier, shortly after 1 a.m., the motorcade that pulled away from Spears' residence was nearly the length of a football field. Along with the ambulance, it included police on nearly a dozen motorcycles and in two cruisers. Two police helicopters followed overhead.
The scene was more controlled than the one earlier this month when Spears also was taken away from her home in an ambulance.
On Jan. 3, police were called to her home when she refused to return her two young sons, Sean Preston, 2, and Jayden James, 1, to ex-husband Kevin Federline, who has custody.
Officers had paramedics haul Spears to a hospital for undisclosed reasons. She was released after a day and a half in Cedars-Sinai Medical Center.
Police also went to Spears' hilltop residence off Mulholland Drive on Monday night after someone reported a swarm of paparazzi trespassing in the singer's gated community. When officers arrived, they didn't see anyone trespassing, police said, but citations were issued for several illegally parked cars.
Late Wednesday night before the ambulance arrived, there were more paparazzi on the scene than usual, and they were joined by several television crews.
Spears has been in a highly public downward spiral since filing for divorce from Federline in November 2006.
Her bizarre antics include shaving her head bald and attacking a car with an umbrella.
Spears' child custody case has become a public spectacle, with her skipping depositions and twice this month arriving at the courthouse but not attending the hearing.
The most recent incident was last week, when Spears showed up at the downtown civil courts building for a chance to persuade a Superior Court commissioner to change his order and allow her to see her sons.
Dressed as if for a cocktail party - wearing bright pink lipstick, sunglasses, shiny gold platform shoes and a very short black dress with a ruffled hem - Spears was driven into an underground garage and then entered the courthouse.
A court spokesman said she got through a security metal detector, then announced, "I want to leave," and returned to her car.