Police in Northern Ireland 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 yesterday.
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."
Dissident republican extremists, opposed to the peace process, have been responsible for the murders of two soldiers, a police officer and a prison guard in recent years.
A group calling itself the New IRA is also believed to have been behind a number of other vigilante-style attacks.
Security sources say the groups are small in number, have little support and are heavily infiltrated.
A number of attacks including a mortar bomb bid on Londonderry's main police station have been thwarted.