A source tells PEOPLE it was the pop star's court-appointed child monitor who called police. "She had already put Preston in the car when Britney locked herself in a room with Jayden," says the source. "The cops came and got through the door and tied her down to a gurney."
Spears, who according to police appeared to be "under the influence of an unknown substance," was rushed to Cedars-Sinai Medical Center, where the source says she will be on a minimum of a 72-hour lock down for mental evaluation.
The evaluation, known as a "5150 Hold," is, according to the medical code, "When any person, as a result of mental disorder, is a danger to others, or to himself or herself, or gravely disabled." The "mobile crisis team" is able to make this recommendation in writing and take a patient be evaluated and treated. (PHOTOS: Britney in the ambulance.)
Spears's WelfareSpears, who has visitation rights with her two sons Preston, 2, and Jayden, 1, was scheduled to give the boys over to Federline at 7 p.m. Thursday.
According to police, officers responded around 8 p.m. to a call "about a custodial dispute regarding Britney Spears's children," says Los Angeles police Officer Jason Lee.
Nearly three hours later, police determined that a court order stated that the singer's children, who were with Spears at the time, were supposed to be with Federline, and a transfer was arranged, according to Lee.
"While officers were on the scene, they observed Ms. Spears under the influence of an unknown substance," Lee added.
LAPD called the incident an "ongoing investigation" and says, "for her own welfare, [Spears] was taken to the local hospital for treatment."
Preston was taken to Federline's home by Federline's assistant, and Jayden was in the ambulance with the monitor, a source says, adding that the boys "won't be back with [Spears] for a long time." Federline took Jayden home early Friday morning. Spears's father Jamie remains at the hospital.
Meanwhile, Federline's lawyer, Mark Vincent Kaplan, arrived at court in an attempt to strip Spears of her visitation rights. Kaplan and Spears attorney Tara Scott met with the judge for about a half hour in chambers.
PHOTOS: See the latest pics from Britney Spears's troubled life.