Bad-boy actor Sean Penn says his opposition to a war in Iraq cost him a lucrative movie role -- but the film's producer, Steve Bing, calls the claim "extortion."

The feud over Why Men Shouldn't Marry erupted yesterday amid dueling lawsuits in Los Angeles.

Penn accuses Bing of "borrowing a page from the dark era of Hollywood blacklisting" by denying him a promised role in the movie, which Bing wrote and was supposed to direct.

But Bing -- a multimillionaire who fathered a child with Liz Hurley -- says he's the object of "an extraordinary extortion attempt by an irrational and irresponsible actor."

Bing says Penn is trying to force him to pay $10 million, even though Penn never had a formal contract to do the movie.

Penn visited Iraq in December, and has complained that President Bush's wish to disarm Saddam Hussein takes "a simplistic and inflammatory view of good and evil."

The movie will begin filming next summer, and is supposed to also star Woody Allen.