Police were attacked with bricks and other missiles while investigating reports of a disturbance at a block of flats in Londonderry in the early hours of Sunday morning.
Three males, aged 16, 29 and 32, were arrested following the incident in Westland Street around 4am.
A PSNI spokeswoman said trouble flared and windows broken after a man was escorted out of one of the flats by other men.
“It’s understood about a dozen other people remained inside the flat. Once out on the street a fight broke out with other people, including those who had previously remained inside the flat, becoming involved.”
She added: “It’s thought that around 30 people were involved.”