Britney Spears has been carried out of her home on a stretcher and taken into custody after police were called in a dispute involving her children.
"While officers were at Britney's home they found that Britney appeared under the influence of an unknown substance," officer Jason Lee told the BBC.
"She is being taken into custody for evaluation," he added.
The 26-year-old singer and ex-husband Kevin Federline are wrangling over custody of their two young sons.
Police were called to Spears' home at 2000 local time (0400 GMT) over a family custodial dispute that they tried to resolve "peacefully by court order", Mr Lee said.
After nearly three hours, Spears reportedly handed over her children, two-year-old Sean Preston and one-year-old Jayden James, to Federline.
Federline has temporary custody of their children because Spears, who has limited visitation rights, failed to comply with court orders.
The pop star's lawyers filed a court motion earlier this week asking to be relieved as her representatives due to a "breakdown" in communication.
Spears and Federline were married in September 2004 and divorced in July 2007.