Seven people were injured, including one man with a serious head injury, French police said.
According to a Northern Ireland police officer accompanying fans at the scene, French riot police moved in after local Nice youths hurled bottles at Northern Ireland supporters in cafes near the central Place Massena.
"About 20 to 30 Nice youths started throwing bottles at the Northern Ireland fans. Some bottles were thrown back, some punches were thrown," the officer said.
"French police arrived to contain the situation." Dozens of riot police were on the pedestrian street after the incidents ended.
Northern Ireland play Poland in Group C in Nice on Sunday. There are an estimated 10,000 Northern Ireland fans and several thousand Polish in the city for the game.
Christian Estrosi, president of the regional council that includes Nice, "condemned" the violence in a Twitter statement.
Earlier, hundreds of English and Russian fans fought battles in the streets of Marseille ahead of their countries' opening match, leaving one Englishman in a critical condition in hospital.