Denis Poroy, Associated Press
Cynthia Lucia, dressed as a Martian, sips a drink at Comic-Con convention in San Diego.

SAN DIEGO — The most passionate Spider-Men, Storm Troopers, Harry Potters and other pop-culture fanatics are headed south for their annual pilgrimage. Comic-Con, the country's biggest comic-book convention, began Thursday at the San Diego Convention Center.

More than 125,000 people a day — many in the costumes of their favorite characters — fill the sprawling seaside space during the four-day convention. They try out the latest video games, seek out collectible books and toys, restock their T-shirt and costume collections and get a preview of anticipated films and TV shows.

The annual convention, now in its 38th year, draws the most avid fans around — the kind who will blog about what's cool and generate online attention that money can't buy. (Just ask the people who cashed in on "Iron Man," which started as a metallic buzz at last year's convention that built all year before eclipsing the $300 million mark at the domestic box office this summer.)

As superheroes go, so goes Comic-Con. And oh, what superheroes have become: Five of the top 20 summer movies are based on comic book characters, including "The Dark Knight," the record-shattering Batman juggernaut that took $158.4 million in its first weekend (beating the previous record set by 2006 Comic-Con darling "Spider-Man 3").

"This summer there's been an embarrassment of riches," said Marvel Studios president Kevin Feige. "Comic-Con is dipping your toe in that water to see how rabid your fan base is."

Big-screen previews contending for early buzz this year include "300" director Zack Snyder's adaptation of the graphic novel "Watchmen"; and "Terminator Salvation," the long-awaited new installment in the franchise. The much-anticipated "Star Trek," on the other hand, has promised no previews or panel discussions, but is still bound to generate ample fanboy chatter.

"Comic-Con attracts a really vocal and discriminating entertainment fan, so it's a great venue to showcase exciting products," said Sarah Greenberg, co-president of theatrical marketing for Lionsgate. "With blogging and the Internet ... people are communicating directly from peer to peer about matters of taste and really quickly you can get a beat on what they're talking about."

"It's an amazing place to find all those word-of-mouth generators in one location," said Rob Friedman, co-chairman and chief executive of Summit Entertainment, which is making its first trek to Comic-Con. The studio is presenting three movies, including its banner Christmas title, "Twilight," a teen-vampire love tale.

"This is completely unique to any audience that you'd be seeing at the normal film-festival circuit," he said. "They're much more fan-oriented. They're much more enthusiastic and not as critical. They come with an eye to enjoy and observe."