The two injured people were rushed to hospital reported regional newspaper Ouest France.
One had a serious head injury while the other had a less serious injury to his hand.
The northern coastal town of Granville holds a carnival every year in the week before Mardi Gras and attracts over 100,000 spectators.
A 27-year-old man was held by police in connection with the attacks, which happened early on Monday morning.
Ouest France also reported that the man was found to be carrying a gun, fireworks and a grenade.
The attack is believed to have happened following a drunken argument.
This year's carnival was the 138th and the event is usually known for its family-oriented atmosphere.