Los Angeles County Sheriff's Department spokesman Steve Whitmore cited "medical reasons" for releasing Hilton, saying she was not free, but "reassigned" to house arrest while wearing an electronic tag.

 Within hours city prosecutors filed a motion seeking her return to jail.

 Jeffrey Isaacs, Chief of the City Attorney's Criminal Branch, said, "the decision whether or not Ms. Hilton should be released early and placed on electronic monitoring should be made by Judge (Michael) Sauer, and not the Sheriff's Department."

 Sauer, who had ordered Hilton not get an ankle monitor, set a hearing for 9:00 am Friday morning, the statement said.

 Hilton, 26, reported to jail late Sunday to serve her sentence, meted out in May for driving with a license that had been suspended after a drunk-driving arrest.

 She was freed early Thursday, fitted with an electronic ankle bracelet and told to serve out the rest of her sentence in her luxury Hollywood home.

 Hilton thanked the sheriff's department and jail personnel "for treating me fairly and professionally."

 "I am going to serve the remaining 40 days of my sentence. I have learned a great deal from this ordeal and hope that others have learned from my mistakes," she said.

 But her early release triggered a wave of anger from civil liberties groups and on the Internet, with people condemning the special treatment allegedly being accorded to the heiress of the Hilton hotel fortune.

 "One rule for the rich and famous and different rules for everyone else," said Najee Ali, director of Project Islamic Hope.

 "On any given day, a thousand female inmates are in custody. Why is one case, out of the blue, treated any different than them?" asked Steve Remige, president of the Association of Los Angeles Deputy Sheriffs.

 Civil rights leader Al Sharpton said in a statement, "This early release gives all of the appearances of economic and racial favoritism that is constantly cited by poor people and people of color."

 According to the gossip website TMZ.com Hilton, who shot to fame after a sex-tape of her and an ex-boyfriend was released on the Internet, had been crying all the time, found it hard to sleep, and had been seen by a psychiatrist.

 "Paris has been crying on the phone, saying she's not sleeping or eating. Paris says her cell is 'freezing cold.' She has three little blankets and no pillow. She's using one of the blankets as a pillow. The room is bright and jail noise echoes through her space," TMZ said, quoting sources.

 Delgadillo said however that Los Angeles jails were equipped to deal with the medical needs of inmates and the decision to release her was "puzzling."

 A powerhouse on the party circuit, the lanky Hilton is widely known for her minidresses, spray-on tan, platinum hair extensions and pouty poses.

 She traded on her image as a so-called "celebutante" -- famous for being rich and famous -- and released a music album, starred in the reality show "The Simple Life."

 She also appeared in films such as "House of Wax," "Pledge This," and the forthcoming "The Hottie and the Nottie."

Agence France Presse