A policeman has been shot dead in what is being described as an "evil deed" by "terrorists" in Northern Ireland.
The killing came as tributes continued to pour in following the murders of British soldiers Sapper Mark Quinsey, 23, and Sapper Patrick Azimkar, 21, on Saturday as they collected a pizza from delivery men at the gates of Massereene Barracks in Antrim.
The dead police officer, who is yet to be named, was gunned down as he and a colleague investigated reports of suspicious behaviour in the Lismore Manor area of Craigavon, Co Armagh, last night.
It is believed the dead man was an experienced officer who had been in the force for more than 20 years.
No-one has yet claimed responsibility for the latest shooting but the Real IRA said they carried out the killings on Saturday.
Politicians are already blaming dissident republicans for last night's attack but Police Service of Northern Ireland (PSNI) chief constable Sir Hugh Orde said it was too early to identify the killers.
Northern Ireland's First Minister Peter Robinson said: "It is with great sadness that I have learned of the murder of a police officer in the Lismore area of Craigavon.
"This officer was serving his community at the time of the incident.
"I unreservedly condemn this evil deed and offer my sincere sympathy to the officer's family circle. The entire police family is in my thoughts and prayers at this time.
"I am sickened at the attempts by terrorists to destabilise Northern Ireland. Those responsible for this murderous act will not be allowed to drag our Province back to the past.
"On behalf of the Democratic Unionist Party and the people we represent, I would urge the Prime Minister and the Chief Constable to take whatever steps are necessary to ensure that innocent life is protected in the face of this terrorist threat across Northern Ireland."
Police said officers were responding to a distress call when they were attacked.
At a press conference early today Sir Hugh said the dead officer was going to the aid of a terrified woman when he was shot.
He added: "We are used to being attacked - But we will not step back.
"It is a sad day for the Police Service of Northern Ireland.
"Today a police officer with his colleagues responding to a call for help from a vulnerable person in the community was gunned down.
"Let me be very clear on a couple of things. First of all, this will not put off me or my officers delivering the service we do to the communities we paid to protect. That will continue unrelenting as it has done in the threat that we have been facing for nine to 12 months.
"We will continue to deliver that service regardless of the threat but mindful of it."
He declined to link the shooting with the murder of the two soldiers.
Asked if the Massereene murders were linked to last night's killing, Sir Hugh said: "I think you are giving (the attackers) credit they ill deserve. I think these are disparate groups, badly infiltrated and indeed many awaiting trial north and south of the border.
"It just reminds us that a small group of people determined to wreck what is huge political progress are becoming more dangerous.
"We are mindful of that and will do our best in every way to bring these people to justice."
Sir Hugh sent his sympathies to the murdered policeman's family and said he was very proud of each and every one of his officers.
Northern Ireland Secretary Shaun Woodward also offered his condolences to the officer's family and said the policeman had been brutally gunned down.
He said the people of Antrim who suffered the loss of two soldiers had shown the way forward with their resilience.
"Those few who committed these cowardly crimes have shown they have the capacity for evil acts but they don't have the capacity to undermine the peace process and the firm political progress."
He said local leaders had shown a unity of purpose in standing by the principles of democracy and the rule of law.
Yesterday, Sinn Fein president Gerry Adams said he supported the efforts of the police to catch the soldiers' killers - believed to be dissident republicans - and urged any members of the republican community with information to pass it on.
"It is the logic of our position and we do have a responsibility to be consistent," he said.
"The popular will in this island is for peaceful and democratic change and that means an end to actions like the killings in Antrim on Saturday night.
"The logic of all of that is that we support the police in the apprehension of those involved."
However, he strongly condemned the decision by Sir Hugh to bring in undercover soldiers from the Special Reconnaissance Regiment to monitor the activities of dissident republicans.
"The Chief Constable made a huge mistake bringing in undercover British Army units," he said.
"You don't understand the history if you don't appreciate that the involvement of these units in the past - totally unaccountable - has led to the same type of suffering as that that has unfortunately been endured at this time by the families of the two British soldiers who were killed."
The Prime Minister visited Massereene Barracks in Antrim yesterday to hold talks with security chiefs.
Gordon Brown said the killers hoped to shatter the political unity at Stormont but had only increased the determination of party leaders to support the peace process.
"They want to send out the message to the world, as I do, that the political process will not and can never be shaken," he said.
"In fact, the political process is now unshakeable."
Meanwhile, a device which appeared to be a pipe bomb was discovered outside Sinn Fein's offices in Cookstown, Co Tyrone.
It was being checked to determine whether or not it was capable of exploding.