THESSALONIKI, Greece (AP) — Authorities in Greece on Saturday detained 313 people in a raid on the university campus of the country’s second-largest city, Thessaloniki, after riot police were attacked by mobs of people hurling more than 100 Molotov cocktails.

Greek police said roving groups of people wearing hoods emerged from the campus of the Aristotle University of Thessaloniki in the predawn hours Saturday to attack a squad of riot police. The unit is usually deployed some distance from the campus to quell any disturbances after all-night parties that take place on university grounds.

Police said all 313 people were released without being charged.

Such attacks against riot police near the university campus are not uncommon but it’s the first time that so many people were detained after such a clash during which an unusually high number of firebombs was used.

Riot police used tear gas and stun grenades to beat back the attackers. One officer was taken to a military hospital for burns to his face and leg while a 21-year-old civilian was treated for respiratory problems, police said.

The university said in a statement that off-campus “extremists” in conjunction with some individuals from within university grounds had committed the attacks. They said an investigation is underway to determine if any students had taken part. They added that no permission had been granted for any party to take place on university grounds.