Eighteen police officers have been injured during serious rioting in the centre of Omagh.
Police say up to 200 people were involved in the violence in the town during which they ripped up paving stones and knocked down a wall to provide ammunition to hurl at officers.
Officers say the incident appeared to start as a drunken brawl outside a nightclub in the Co Tyrone town's Castle Street at 2am.
However, it developed into more extensive violence lasting two hours during which 15 people including juveniles were arrested for public order offences.
Local Police Service Inspector Paul Douglas has described the trouble as "mindless violence".
He says an extensive inquiry had been mounted and all evidence will examined including footage from CCTV cameras.