EXCLUSIVE

Emma Roberts was arrested for a domestic violence incident in Canada -- after allegedly bloodying her boyfriend, Evan Peters ... TMZ has learned.

22-year-old Emma -- Eric Roberts' daughter and Julia Roberts' niece -- was taken into custody early July 7 in Montreal after getting in an altercation with Peters ... who stars in "American Horror Story."

According to Montreal law enforcement sources ... someone called police to report a fight in Evan and Emma's hotel room.

We're told when cops arrived ... they observed Evan with a bloody nose. Emma was immediately arrested. One law enforcement source tells us Evan also had a bite mark.

Emma was released hours later because 26-year-old Evan did not want to press charges.

Sources close to Emma tell TMZ ... Emma and Evan were hitting each other and she was arrested because he had obvious physical injuries. The sources deny Emma bit Evan.