By Mike Collett-White and Silvia Aloisi

VENICE (Reuters) - Ang Lee has walked off with another top award at the Venice film festival, and surprised critics are wondering how he did it.

"Lust, Caution" ("Se, Jie") won the Golden Lion late on Saturday, two years after the Taiwanese director scooped the big prize for gay cowboy drama "Brokeback Mountain."

In 2005 he was a popular winner. In 2007 he was not.

Reporters and critics in the press room, watching the closing ceremony beamed live on a big screen, booed when Lee's Golden Lion was announced, and again, more loudly, when Hollywood star Brad Pitt was named best actor.

"In all of the pre-award speculation, nobody had thought about Ang Lee's film, not even for one of the lesser awards," Natalia Aspesi wrote in La Repubblica newspaper on Sunday.

At a post-verdict news conference, jury president Zhang Yimou offered no explanation of the panel's decision to give the Golden Lion to Lee's film.

Reviews of "Lust, Caution" were generally negative, arguing that at 156 minutes it was much too long. The slow narrative, set in World War Two Shanghai, is punctuated by explicit and sometimes violent sex which Lee hinted was real.

The Hollywood Reporter's verdict was: "Ang Lee's lugubrious spy epic ... brings to mind what soldiers say about war: that it's long periods of boredom relieved by moments of extremely heightened excitement."

The decision meant that a festival broadly praised for its film selection and organisation ended on a slightly sour note.

Although no strong favourite emerged from the 23 films in competition in Venice, critics agreed Tunisian-born Abdellatif Kechiche's "The Secret of the Grain", about an Arab family living in France, would be a worthy winner.

In the end a disappointed-looking Kechiche walked away with a jury runner-up prize, as did U.S. film maker Todd Haynes for his conceptual biopic of Bob Dylan called "I'm Not There".

Also in the frame in the runup to the awards were two films about the war in Iraq.

In Paul Haggis's "In the Valley of Elah", Tommy Lee Jones won praise for his performance as a war veteran whose son is murdered after returning from Iraq, and Brian De Palma's "Redacted" shockingly recreated abuses in Iraq by U.S. soldiers.

"MYSTERY" AWARD TO PITT

More controversial than the Golden Lion for Lee was the best actor prize for Pitt, who starred as the fabled outlaw in "The Assassination of Jesse James by the Coward Robert Ford."

Critics said the decision was particularly baffling because Pitt's co-star in the movie, Casey Affleck, was widely considered to have stolen every scene, while Jones and Michael Caine in Kenneth Branagh's "Sleuth" were also frontrunners.

More popular were the Silver Lion for best director to De Palma, whose brutal film stunned audiences, and the best actress prize for Cate Blanchett.

In a daring piece of casting that appears to have paid off, the Australian-born actress was one of six performers to play Dylan in "I'm Not There", and arguably the most convincing.

Zhang sought to soothe one journalist who challenged the jury's decisions, saying that "all the force and power of heaven" would not be enough to guarantee a decision that kept everybody happy.

But the jury appears to have bent over backwards to appeal to as many tastes as possible by awarding the jury runner-up award to two films instead of one, and handing Russian director Nikita Mikhalkov a special Golden Lion for his work.