The victim was taken to a nearby hospital and treated for a minor cut near his eye, police said. The assailant -- in a blood spattered "Harry Potter" shirt -- was detained by fellow audience members waiting for a Universal Studios panel discussion Saturday night, the San Diego Union-Tribune reported.
The audience booed and yelled at the suspect as he was lead away in handcuffs by police, the newspaper said.
The two men, in their 20s, were apparently arguing about seating when the assault occurred, police said.
Police said violent incidents at the pop culture and comic book convention are rare. The first Comic-Con in San Diego was in 1970. More than 125,000 people passed through the doors during this year's convention, which ends Sunday.