French police arrested 101 Roma fans, some with weapons, in Nice on Tuesday, a day before the teams were due to meet in the Europa League, the local prefecture announced.
A separate source told AFP that the Italian supporters were carrying sticks, shovels and knuckle dusters around the city centre on the French Riviera.
The prefecture said that more than 400 police officers will be present for Wednesday's group stage match.
"Since yesterday, more than 200 personnel, including two mobile units, were deployed in the city centre of Nice to prevent all types of public disorder and proceeded with the arrest of 102 individuals identified as Roma ultras, carrying weapons," it said on Wednesday, before later revising the number of arrests down to 101.
"These individuals were taken into police custody," it added.
The local authorities have banned Roma fans from displaying visible signs of their allegiance in the city between 1400 BST (1500 local time) Tuesday and 1100 BST on Thursday.