Two handguns, two hand grenades, over 50 rounds of ammunition and more than a kilogram of plastic explosives were recovered in the three searches of the Creggan area of Londonderry/Derry late Thursday.

Unionists who favour British rule in Northern Ireland call the city Londonderry. Nationalists who want reunification with Ireland use Derry.

Officers in Northern Ireland have been subject to sporadic attacks by dissident republicans and a data breach last month sparked heightened security concerns.

"These items are military grade and we believe the intention was to mount attacks in an attempt to kill police officers," the Police Service of Northern Ireland (PSNI) said in a statement.

"The significance of this recovery cannot be underestimated."

Two men and a woman had been arrested under the Terrorism Act and were being held in custody, the force added.

Following the searches in the city and the nearby town of Strabane, police and their vehicles came under "sustained violent attack with heavy masonry and petrol bombs", the PSNI added.

Sixteen officers were hurt with injuries including burns, head injuries and possible broken bones.

Police in Northern Ireland were targeted frequently during the 30 years of conflict over British rule in Northern Ireland, which claimed more than 3,000 lives before the signing of 1998 peace accords.

"Not only was the intention here to kill and maim, but there is a total disregard for the wider community," police added.

"These items were being stored in a residential area in close proximity to a children's play area and any attack that would have been mounted would have taken place within our communities."

In March the terrorism threat level in Northern Ireland was increased to "severe" in response to an assassination attempt on senior police officer John Caldwell, later claimed by the dissident republican group the New IRA.

Last month, the then-PSNI chief constable Simon Byrne told reporters information on police officers and staff that was mistakenly released in a major data breach was in the hands of dissident republicans.

"It is therefore a planning assumption that they will use this list to generate fear and uncertainty as well as intimidating or targeting officers and staff," he said.

The force said a week earlier it had published in error the names and ranks of roughly 10,000 officers and staff in response to a routine freedom of information request.