If you happen to see Optimus Prime walking across the Massachusetts Avenue bridge, or Pikachu hanging out in the Back Bay area, don’t call the police — it’s just attendees gearing up for the annual Anime Boston convention.

“There’s a lot of activity going on,” said Tuan Pham, public relations director for the event. “We have every room in the Hynes Convention Center filled with something.”

The fourth-largest anime convention in the country, Anime Boston will draw close to 20,000 people over the course of three days.

Playing heavy on “cosplay,” where anime fans dress up as their favorite characters, Pham said the weekend is a “cacophony of sounds and excitement,” featuring fashion shows, industry panels and a shopping area for fans to spend thousands of dollars on unique merchandise.

“People usually go in with $600 and leave with nothing,” said Pham of the die-hard fandom surrounding the anime culture.

Each year, Pham said, some 35 percent of those who attend dress up in intricate outfits. “People spend hundreds of hours on their outfits prior to the event,” he said.

Last year, one fan constructed a homemade Optimus Prime costume.

“It was plexiglass, but it could transform,” explained Pham. “It would snap and click together.”

Massachusetts Institute of Technology student James Duyck, 19, joined the hordes of anime fans that lined up Thursday in full character attire. Duyck said he spent his recent spring break working on an outfit specifically for the convention.

“My costume is a long robe and suit of armor, with shoulder pads,” said Duyck, who is the treasurer of MIT’s anime club. “This is the first time I am going, so I’m pretty excited.”

Both Pham and Duyck agreed:?It’s the camaraderie of like-minded, costumed convention-goers that makes the weekend a hit.

“Everyone has something to talk about and share,” said Pham. “People look forward to next year’s Anime Boston the day after it ends.”

PAX East puts gamers in one room

While Anime Boston draws a costumed crowd to the Hynes, the Boston Convention Center will be mobbed with gamers for this year’s PAX East expo.

Patrick Moscaritolo, president and CEO of the Greater Boston Convention and Visitors Center said PAX East — a three-day festival for tabletop, video game and PC gamers — will bring an international crowd and dump millions into the Massachusetts economy.

“It’s just a great piece of business for a city, in particular this time of year,” he said.

PAX East is the largest East Coast gaming get-together. In February, the Massachusetts Convention Center Authority signed a 10-year deal with the event’s hosts to keep it coming back to the Hub.

From geek to chic

Anime Boston 2012 will take over the Hynes with three days of panels and events.

How to Survive a Robot Apocalypse: Find out what to do once robots take over the world.

Phoenix Ball: Spend the evening swaying to anime and video game favorites, dressed to the nines. Can’t dance in costume? Don’t you worry:?They will teach you how.

Hyakunin Kuizu: It’s the Japanese version of Family Feud.

Extreme Geek: This 18-and-older event is for “anyone who loves a little bit of loony immaturity.”