Kevin, another fan who declined to give his surname, said of the expected Scudetto party: "It will be chaos, mayhem.
Authorities also banned car and scooter traffic from the city centre in the hope of minimising risk to public order.
"I've been waiting for the Napoli Scudetto since I was small," said a teenage girl out with two friends, her cheeks painted Napoli blue.
"It's a victory for the entire city, it's wonderful," she added, declining to give her name.
In Udine, there were post-match scuffles as Napoli fans ran onto the stadium pitch to celebrate with players, but were met by hostile home supporters armed with belts and batons, the ANSA news agency said.