UPDATE 2-'Watchmen' falls short of expected box office take

* "Watchmen" opens at No. 1 with $55.7 million

* Costly superhero saga falls short of expectations

* Earns a solid $27.5 million in 45 territories

(In paragraph 6, updates with foreign sales for "Watchmen")

LOS ANGELES, March 8 (Reuters) - "Watchmen," an unorthodox superhero movie that took two decades to reach the big screen, took the No. 1 spot at the weekend box office in North America on Sunday, but fell a bit short of expectations.

The adaptation of a cult comic book series sold an estimated $55.7 million in tickets in its first three days, distributor Warner Bros. Pictures said, becoming the biggest opening of the year.

But pundits had expected an opening in the $60 million-plus range, and the tally was considerably lower than the $71 million start two years ago for "300," the previous film from "Watchmen" director Zack Snyder. The ancient battle epic holds the record for a March opening. "Watchmen" ranks at No. 3.

"Our expectations were met," said Dan Fellman, president of domestic theatrical distribution at the Time Warner Inc (TWX.N)-owned studio.

He said the film's 161-minute running time inevitably affected business, restricting theaters to one main evening screening. Male moviegoers accounted for about two-thirds of the audience, with the "sweet spot" aged between 17 and 35, Fellman said.

Internationally, where the film was released by Viacom Inc's (VIAb.N) Paramount Pictures, "Watchmen" earned a respectable $27.5 million from 45 territories. Top markets included Britain with $4.6 million and France with $2.5 million. The only major country still waiting is Japan, where it will open on March 28.

OBSCURE ACTORS, CHARACTERS

"Watchmen," which cost about $120 million to make, revolves around a team of crime fighters targeted in a dastardly plot with dangerous implications for mankind.

A relatively unknown cast plays a similarly obscure lineup of characters, including the vigilante Rorschach (Jackie Earle Haley), the naked blue giant Dr. Manhattan (Billy Crudup), and the occasionally topless Silk Spectre (Malin Akerman).

Top critics were largely underwhelmed by "Watchmen," according to Rotten Tomatoes (www.rottentomatoes.com), a web site that aggregates reviews.

The project is based on the sprawling 1980s "Watchmen" comic books by Alan Moore and Dave Gibbons, which were long considered unfilmable because of their multiple characters, violence, digressions and abundance of dialogue.

That did not stop studios including Twentieth Century Fox and Paramount from attempting adaptations. Warner Bros. came aboard in late 2005, and brought on Snyder who was working on the effects-heavy "300" at the time.