Chris Evans, Mark Ruffalo, Chris Hemsworth, Jeremy Renner and Robert Downey Jr. were on hand Saturday with other cast members including Samuel L. Jackson and Cobie Smulders.
"It just keeps getting better," Downey told the boisterous crowd of 6,500 assembled in the San Diego Convention Center. "I'm so excited we're all here."
The footage kicked off with the Marvel superhero team comically attempting to pick up Thor's mighty hammer off a coffee table.
"If I lift it, do I get to rule Asgard?" Tony Stark (Downey) wondered while trying to wield Thor's hammer -- both with and without one of his Iron Man gauntlets.
The teaser quickly launched into a barrage of action-packed imagery, including Hulk (Ruffalo) battling a giant robot resembling Iron Man and Thor grabbing Stark by the throat. The footage ended with a bleak scene of all the Avengers fallen with a haunting close up of Captain America's (Evans) shield split down the middle.
Jackson, who most recently starred in "Captain America: The Winter Soldier" with Evans, took a moment during the celebration to appreciate the fans.
"It makes me feel like going out and making movies for the rest of you," he said of coming to Comic-Con. "We make movies because we want to entertain you guys, and I make movies because I want to see myself in them."
"Avengers" director Joss Whedon wasn't present at Saturday's unveiling because he recently underwent knee surgery, nor was Black Widow Scarlett Johansson (currently expecting her first child with fiance Romain Dauriac), who sent along a video message.
The original "Avengers" cast was joined on stage inside Hall H by newcomers Elizabeth Olsen, Aaron Taylor-Johnson, Paul Bettany and James Spader, who personifies the villainous mechanical force Ultron in the sequel.
"I play an 8-foot robot in this movie," Spader joked. "I've always played humans up until this point."
In the trailer, Ultron takes a few different forms, from a wobbly 'bot to slick metallic general. At one point, he tells the Avengers that he has no strings.
"Joss really gets these big, big, big ideas," said Ruffalo backstage after the panel "There is a discussion that's coming up about artificial intelligence and the ethics of artificial intelligence I think that's dealt with in the movie. We're kind of on the threshold of that -- the movie 'Her' -- all of this technology is just about ready to hit the world. But he's also talking about our ability to destroy the world. And it's a real thing. We're there."
The presentation ended with a brief appearance by Josh Brolin, who voices the alien baddie Thanos in both "Age of Ultron," which is set for release May 1, 2015, as well as "Guardians of the Galaxy," the cosmic Marvel romp that comes out Friday. Brolin came on stage wearing a foam Infinity Gauntlet, the powerful artifact depicted in Marvel lore.