Police are stepping up patrols and increasing check points in a bid to counter the dissident republican threat.
Dissidents have been blamed for a spate of recent attacks including the discovery of two partially exploded bombs at an Army reserve base in Londonderry on Monday.
Last week, police in Belfast escaped injury when a blast bomb type device was thrown at their vehicle and detonated in mid air.
Assistant Chief Constable Stephen Martin described the threat as “severe”.
He said: “This is not a decision we take lightly and we will endeavour to ensure that the balance of how we police this threat is right, as well as trying to keep disruption to a minimum. However, please be assured we will always prioritise public safety.
“There is no perfect security solution to terrorism. Long term success against the small group of people intent on causing disruption and harm to the community needs a combination of policing, community and political efforts and it is vital we all play our part.”
The First and Deputy First Ministers have stressed that peace is “the only way forward” in the wake of a string of violent incidents, including the murder of an ex-IRA man.
Former IRA commander Gerard ‘Jock’ Davison was gunned down in a republican district on the edge of Belfast city centre.
The PSNI does not believe dissident republicans were behind the killing.
Speaking about the latest violent incidents, Peter Robinson and Martin McGuinness issued a joint statement, which read: “This murder and these attacks and all other forms of violence are to be condemned. Attacks such as these are designed to take us back and they will fail.
“We must create a community where everyone feels safe and we will not be deterred by the bullet or the bomb and urge anyone with any information to bring it to the PSNI.
“Peace and democracy is the only way forward.”