Gardaí pounced on several residences in Letterkenny on Tuesday.
Three people remain in Garda custody following a series of raids in Letterkenny.
A total of eight people were arrested on Tuesday as part of a major swoop on drug-related activity.
Armed Gardaí entered residences in the Lower Main Street and Meadowbank areas of Letterkenny in a planned operation.
Those arrested were taken to various Garda stations in the county for questioning.
Four of those arrested have been charged with offences believed to be relating to drug-related crime.
Another has been released without charge and a file is being prepared for the Director of Public Prosecutions (DPP).
Three others were still in Garda custody being quizzed on Wednesday afternoon, a spokesperson for the Garda Press Office told Donegal Live.
Gardaí have confirmed that a quantity of drugs was seized by officers in the operation.
Uniformed Gardaí and detectives were aided by members of the Armed Response Unit in the operation, one of the largest conducted in recent times.
Drug-related crime in Donegal has risen sharply.
In 2016, there were 248 controlled drug offences in the Donegal Division. That figure rose by a staggering 120 per cent by the end of 2020.
In the whole of 2020, there were 196 offences for drug-driving in Donegal. By the end of March, 2021, there were already 178 offences for drug-driving recorded in the county.
In early 2021, €2.8million worth of cocaine was located near Kilmacrennan. Some 41kgs of the substance was located by officers from the Milford District Drugs Unit.
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