Police in Derry used CS spray after coming under attack by a gang as they went to the aid of an assault victim earlier today.
Police said the incident happened shortly before 2 a.m. when officers intervened to stop an attack on a young man in Waterloo Place.
As the officers were attempting to withdraw, they were attacked by a crowd of around 100 youths, a police spokeswoman said.
A 16-year-old man was arrested for disorderly behaviour and CCTV footage is being examined in a bid to identify others involved.
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Twenty minutes earlier, officers had used CS spray to help quell a riot in the nearby Victoria Market car park.
It was also used to prevent another attack on a young man, who was being assaulted by up to 30 youths.
PA