Security forces in Northern Ireland were on full alert today amid fears that dissident republicans could unleash an Easter offensive.
Just three weeks after the murder of Constable Ronan Kerr in Omagh, police warned that terrorist groups could be intent on trying to murder more officers in the coming days and weeks.
A police spokesman said: "Dissident terrorist groups are continuing to identify officers and target them with the single objective of killing them. And in so doing, their reckless actions will also put the lives of our wider communities at risk."
The new warning came as thousands of mainstream republicans supporting Sinn Fein and the peace process prepared to hold dozens of commemoration parades throughout Ireland to mark the 1916 Rising.
Police said the public needed to be on their guard.
Patrols and on-the-spot security checkpoints are to be set up across Northern Ireland.
A spokesman said: "We are taking these steps to keep communities and officers safe. We would not do this unless it was not absolutely necessary to protect life."
Three men were arrested yesterday when police stopped a car in Keady, Co Armagh.
Police described the arrests as "significant" and it is understood a number of undisclosed items were taken away for examination.
The men were detained as part of a security operation which also involved police in the Republic.
Meanwhile, a 33-year-old man from Omagh, Co Tyrone, will appear in court later to face a series of terrorist charges after being questioned by detectives investigating Constable Kerr's murder.