Two protesters, a man and a woman, were in the audience before managing to make their way into the main ring during the "novillada", a series of fights involving young bulls.
One bull charged at the man who, according to police a received "a long but not deep" injury from its horns.
The protester, in his 30s, "was very lucky" that he was not properly gored and was only lightly injured, another source said.
He was taken to Carcassone hospital for examinations. His female companion was not inured and was arrested by police.
Earlier two other protesters had briefly hung a banner saying "Stop Bullfighting" from the ramparts of the medieval French town.
Bullfighting is banned in most of France but is allowed in some southern regions where it is protected as part of local traditions.
Defenders of bull fighting say it has huge cultural importance, embodying traditions dating back hundreds of years but its popularity has been steadily waning in recent years.