Three men have been arrested and five assault rifles and three handguns have been seized in overnight raids in Derry.
It was part of a major police operation targeting the vigilante group Republican Action Against Drugs.
PSNI Chief Constable Matt Baggott said the operation was aimed at tackling punishment shootings in Derry.
Police said a total of eight firearms were taken after homes in the city were searched.
The men who were arrested are being questioned at the PSNI's serious crime suite in Antrim.
The PSNI said officers came under attack from youths throwing stones and petrol bombs at several locations.
Since 2008, Republican Action Against Drugs has murdered one man and shot more than 40 others.
In 2011, Andrew Allen from Derry was shot dead in a house in Buncrana in County Donegal.