Police in Northern Ireland have warned about the possibility of violent dissident republican disorder over the Easter holiday.
“Strong” intelligence has been received that suggests terror attacks could be launched on officers in Londonderry on Monday.
The warning comes ahead of US President Joe Biden's much-anticipated visit to Belfast on Tuesday.
Mr Biden's trip, which will also include events in Dublin, County Louth and County Mayo, will have a strong focus on the 25th anniversary of the Good Friday Agreement, which falls on Easter Monday.
Easter Monday is the day dissident republicans traditionally mark the anniversary of the Easter Rising rebellion against British rule in 1916, with a parade set to take place in Londonderry.
Police Service of Northern Ireland (PSNI) said it will move officers to frontline duties on the bank holiday to counter any threats, in a strategy which has not been used “in years”.
Chief Constable Simon Byrne said this reflected the "exceptional circumstances" ahead of this Easter weekend.
Speaking in Belfast, Irish Tanaiste, Minister for Foreign Affairs and Minister for Defence, Micheal Martin, condemned the threat of a terrorist attack as "criminality in its worst form" and said it was "very evil people who are contemplating this".
MI5 recently raised the terrorism threat level in Northern Ireland to severe, meaning an attack is highly likely.
It followed the gun attack on senior detective John Caldwell in County Tyrone, who has been left with life-changing injuries.
Police have blamed the New IRA for the attack.
"It's going to be a really significant weekend for the PSNI," Assistant Chief Constable Bobby Singleton said in Belfast.
"There is also very strong community intelligence specifically coming forward in respect of Monday's events in Derry/Londonderry and a real concern that there may be attempts to draw police in to serious public disorder and to use that then as a platform to launch terrorist attacks on police as well.
"So going into our operation that's something that is very clearly right at the forefront of my mind, the minds of the commanders that will be delivering that and of course our officers as well."
Mr Singleton said while dissident republican intent to kill police officers remained the same, he said officers were concerned they may use public disorder in Londonderry as a platform to launch attacks.
When asked about whether guns or explosives could be used to target police in Londonderry, Mr Singleton said: "We've seen that in the past and, on that basis, we have to be prepared for that and we will be prepared for all eventualities on Monday."
"I would condemn any attempts by anybody to injure or to attack the PSNI," Tanaiste Mr Martin said on Thursday.
"An attack on the PSNI is an attack on all of us on this island. The police are essential to the security of our people and the PSNI, in my view, are doing a remarkable job under very challenging circumstances.
"I was listening to PSNI personnel this morning talking about their work in communities.
"The tremendous progress that has been made in terms of the transformation of policing in Northern Ireland is one of the great dividends of the Good Friday Agreement.
"I'm resolute in my condemnation of such attacks, as we witnessed on John Caldwell.
"I think it's very evil people who are contemplating this - this is criminality in its worse form."
Briefing the Policing Board in Belfast, the Mr Singleton said the overall tone in both the operational threat and the resourcing picture facing the PSNI is "stark and sombre".
"We are now dealing with a severe terrorist threat, which means that an attack is highly likely right across Northern Ireland.
"The thing to stress is the main focus of these attacks continues to be police officers, both on and off duty, and their families.
"It will also include prison officers and military personnel.
"The style of attack that we are dealing with and trying to frustrate is gun attacks and bomb attacks on these people by a small number of determined dissident terrorists.”
New IRA suspected in attack on police officer in Northern Ireland - in pictures
Mr Byrne said the force had initiated what he called “Operation Inspire level four”.
"What that means is that in some cases operational officers across the service have moved to 12-hour shifts and officers in a number of non-frontline roles will move to support the collective effort to focus on preventing further terrorist outrage and actually increasing our patrol visibility across the country.
"This is the first time in a number of years we have had to activate this plan.
"It is a reflection of the quite difficult and exceptional circumstances we find ourselves in."
Mr Byrne said the plan would remain in place for around 10 days and would then be reviewed.
The Chief Constable said, "If that wasn't enough, we are on the cusp of the Easter period which is traditionally quite busy. There is a significant policing operation in place.
"There are 90 notified parades and events taking place between Good Friday and Easter Tuesday.
"In addition our planning is based on experience which takes into account the risk and likelihood of policing non-notified parades.
"This may include the display of paramilitary uniform and paramilitary shows of strength and we will have resources in place to deal with that.
"We are also assuming attempts will be made to draw us into situations which may likely cause disorder over the next few days."
PSNI Assistant Chief Constable Chris Todd said there was no specific intelligence that the anniversary of the Good Friday Agreement was acting as an additional motivation for dissident republicans to launch attacks.
"We plan for the worst and we hope for the best to be quite frank, we will respond to the intelligence as it develops, we have no such intelligence that would support that at the moment," Mr Todd said.
