Britney Spears was placed on lock down for a mental evaluation Friday after the pop star locked herself in a room with one of her children at her L.A. home and refused to hand him over to ex-husband Kevin Federline.
A source tells PEOPLE the pop star's court-appointed child monitor 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.
Spears, 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 the hospital 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 his sons home early Friday morning. Spears's father Jamie remains at the hospital.
Meanwhile, Federline's lawyer, Mark Vincent Kaplan, is due in court at 8:30 a.m. to attempt to strip Spears of her visitation rights. Attorney Tara Scott from the law firm of Trope and Trope will represent Spears even though the firm has asked to withdraw as counsel.