The PSNI arrested four men after seizing a quantity of suspected bomb-making equipment in Lurgan, Co Armagh this morning.
The men were arrested and suspected bomb-making materials, including fertilizer, were seized during the planned search operation in the Co Armagh town.
Police said the estimated 100 kgs of homemade explosives that were seized were being prepared for an imminent bomb attack.
Petrol bombs hit PSNI vehicles in Armagh today after the arrests. PA
Supt Alan Todd confirmed this afternoon that dissident republicans were the main suspects.
He described the quantity taken during a planned search of premises close to a housing estate as capable of the "carnage in our times wreaked in the past."
"Material of that sort is by its nature unstable," he said. "The people preparing these things do so with the intent to use it as soon as possible.
"The device was being constructed for immediate use. But more importantly, it means that unstable explosive substances, ready for use, were being prepared in what is a busy residential area behind a commercial premises.
"The risk that provides to the local community is very high and we are very happy police have been able to act."
The men, aged 22, 26, 36, and 46, were arrested under the Terrorism Act. Three were arrested early this morning and one later at the scene on Antrim Road, close to Lurgan's railway line.
A PSNI spokeswoman confirmed the investigation was in relation to dissident republican activities.
The SDLP's Dolores Kelly congratulated the police on the operation and said "Those behind this bomb making activity are an enemy to us all and they are enemies to political progress. But their dirty plans and dangerous activity will not hold this process to ransom," she added.
British Army bomb disposal experts attended the scene.
Later police were attacked with petrol bombs near the scene of the security operation.
Masked men were spotted in the area and crowds also threw stones, bottles and paint bombs at officers and vehicles. No injuries were reported but a number of police vehicles were believed to have been damaged in the attack.
Fire chiefs confirmed that a shed at the breakers yard, which contained gas cylinders, had been deliberately set alight.
