Britney Spears is on lockdown for a mental evaluation after she 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, PEOPLE reports.

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, 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.