The police have warned of the potential of public disorder linked to dissident republicans over the Easter period.
Chief Constable Simon Byrne has told a meeting of the Policing Board that the trouble could be an attempt to draw officers into gun or bomb attacks.
MI5 recently raised Northern Ireland’s terrorism threat level to severe, meaning an attack is highly likely.
Events are being held to mark the 25th anniversary of the signing of the Good Friday Agreement in 1998.
The deal ended 30 years of violent conflict in Northern Ireland, known as the Troubles, which cost the lives of more than 3,500 people.
Mr Byrne said the PSNI had made temporary changes to shifts to put more officers onto frontline duties.
In February senior detective John Caldwell suffered life-changing injuries when he was shot multiple times at a sports complex in Omagh, County Tyrone.
Police believe the dissident republican group the New IRA was behind the attack on the off-duty officer.
Speaking after the Policing Board meeting, Assistant Chief Constable Bobby Singleton said the PSNI had “very strong community intelligence” that attacks were being planned in Londonderry.
He said officers had to “be prepared for that and we will be prepared for all eventualities on Monday”.
Speaking in Belfast later on Thursday, the Tanaiste (Irish deputy PM) Micheál Martin said the possibility of an attack on police represented “criminality in its worst form”.
“I think it’s very evil people who are contemplating this,” he said.
Earlier Mr Byrne also said resources would be further challenged by the visit of US President Joe Biden next week in relation to the Good Friday Agreement anniversary, which actually falls on Monday.
About 300 officers will be drafted in from other UK forces to help out.
The cost of the security operation around the presidential visit, anniversary events and Easter has been put at £7m.
The Good Friday Agreement
In his latest report to the Policing Board, Mr Byrne said Det Ch Insp John Caldwell remains in a “poorly condition” in hospital.
The chief constable revealed that the PSNI had created a “bespoke CCTV viewing hub” as part of the investigation into the attempted murder.
The hub is staffed by 30 detectives who are “working through upwards of 100,000 hours of CCTV obtained through our enquiries”.
Mr Byrne also gave some more details about the extent of the investigation in to the gun attack, confirming that:
- 15 arrests have been made, three of which related to a claim of responsibility for the shooting
- 22 searches of premises and land have been conducted to date
- More than 300 witnesses have been spoken to by PSNI officers
As well as the threat from dissident republican paramilitaries, Mr Byrne’s report also addressed the problems posed by ongoing violence within loyalist groups.
He said the PSNI was “currently dealing with the impact of a violent feud between criminal drug gangs” which he said were previously linked to the Ulster Defence Association (UDA), a loyalist paramilitary organisation.
The feud has led to gangs of masked men on the streets of Newtownards and several homes in north Down attacked by petrol bombs, bricks and at least one pipe bomb over the past few weeks.
As recently as Wednesday night, small groups of masked men were reported in several areas of Newtownards and police said a number of threats were made to people, telling them to leave the town.
“We have carried out a number of targeted search operations and arrested 10 men at the time of writing, two of whom have been charged, appeared in court and are now remanded in custody,” Mr Byrne said.
“The policing operation… will continue to work to stop these reckless attacks which put our Community at risk and have no place in society.”
“Set against the backdrop of this demanding operational context, I remain gravely concerned about the stark budget challenge we face as an organisation,” he added.