Roman Polanski's lawyers vowed to fight his extradition to the United States over a three-decade-old child sex case and called his surprise detention in Switzerland "illegal".

Support from the movie industry for the 76-year-old Polish-French film director also grew as top film directors declared themselves "astonished" at his arrest for the 1977 case.

"Filmmakers in France, in Europe, in the United States and around the world are dismayed by this decision," said a petition organized by SACD, which represents performance and visual artists.

"It seems inadmissible to them that an international cultural event, paying homage to one of the greatest contemporary film-makers, is used by police to apprehend him."

Polanski case: key dates. French Foreign Minister Bernard Kouchner said the case was a "bit sinister" and that he had asked the United States to drop the charges. A Swiss newspaper said the country should be "ashamed" of the "trap" it laid for the Oscar-winning director.

Polanski, who achieved international acclaim for movies such as "Rosemary's Baby", "Chinatown" and "The Pianist", was detained as he arrived on Saturday to receive a lifetime achievement award at the Zurich film festival.

The French consul general in Zurich said after visiting Polanski in a Swiss prison that the film-maker was being well treated following his arrest on a US warrant.

"Mr Roman Polanski is being well treated. He thanks the many people who have shown support through the press and public opinion," the consulate said in a brief statement following the visit by Consul General Jean-Luc Faure-Tournaire.

Also visiting Polanski was Poland's ambassador to Switzerland, Jaroslaw Starzyk, who also found him to be "in good condition," Jolanta Chojecka, chief of the consular section at the Polish embassy in Bern, told AFP.

Swiss authorities said they were awaiting an extradition request from the United States over the case in which he admitted having sex with a 13-year-old girl. The victim has since joined defence lawyers in urging for the case to be dismissed.

Los Angeles prosecutors confirmed Monday they would file a warrant seeking Polanski's return to the United States, noting they had 40 days to prepare it.

"We will prepare an extradition warrant and it will be sent to the Justice and State Department and it will go through the diplomatic channels," Los Angeles County District Attorney spokeswoman Sandi Gibbons said.

But Polanski's French lawyer, Herve Temime, said the director rejected any prospect of extradition.