Britney on 72-Hour Mental Lockdown

Britney Spears was placed on lockdown 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 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.

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