For some, it's a fun dress-up day without any judgment because everyone is a fan.
"Growing up as a geek, I usually got picked on, but now this is our realm," said San Francisco resident Michael Bachman.
A San Diego resident dressed up as Captain America said thousands of people have come up to him for pictures.
"It's just people who are brought together by a common love of entertainment and comic books and it's a great time," said Lisa Gregorian of Warner Brothers Television.
Movies are a major part of Comic-Con and big names like Eva Mendes and Seth Rogen made it a point to join the festivities.
Comic-Con runs through Sunday.
