A police officer was shot dead in Craigavon last night, just 48 hours after Real IRA dissidents shot dead two British soldiers.
The incident happened in the Lismore Manor area andfollows an attack on Saturday when two soldiers were shot dead by dissident republicans as they collected a pizza delivery at their base in Antrim.
A PSNI spokesman said: "We can now confirm that one police officer has died following a shooting incident that occurred shortly before 9pm in the Lismore Manor area of Craigavon."
Police have been warning of a heightened threat from dissident gunmen for some time.
Police later said two squad cars were involved in separate collisions on the Tandragee Road and Lake Road as they travelled to the scene last night.
A manhunt was already under way after the two soldiers were killed outside Massereene Barracks on Saturday night.
Politicians across the community condemned that action as an effort to destabilise the power-sharing administration and this latest incident will prompt further concern.
Politicians from across the political divide were last night quick to condemn the shooting and to blame dissident republicans opposed to the peace process for the killing.
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."
Democratic Unionist Party (DUP) member of parliament for the Upper Bann constituency where the shooting took place, David Simpson, linked the shooting to the dissident republican murder of the two soldiers shot dead at Massereene barracks on Saturday.
He said the political process was under attack from terrorists intent on causing mayhem.
"What we have seen over the last 36 hours is a deliberate and sustained effort by terrorist murderers to try and drag Northern Ireland back to the worst days of Ulster's past," he said.
"I utterly and completely condemn the criminals responsible for this latest outrage and I hope that the vermin responsible for it are brought to justice immediately.
"Events such as the murders at Massereene and this latest terrorist atrocity show us all the need for a swift, co-ordinated and ruthless security and government response."
Lord Maginnis, UUP spokesman, said those behind the attacks had barely any "grass roots" support but were getting "an opportunity" to publicise their cause.
He told the BBC: "We have a chief constable who has warned us and warned us and warned us about the danger of dissident IRA. That has put pressure on the leaders of dissident IRA, what I call rebel IRA, to have a big show to kill those soldiers at the gates of Massereene barracks, to kill this policeman in Lurgan.
"I just feel we have just got it so wrong in terms of how we are dealing with the potential for terrorism from this small group of people.
"As we publicise them we give them almost the motivation to carry out this type of attack. We are going to have a period now when in fact they will go for soft targets whether that be politicians, police or soldiers.
"These evil people are getting an opportunity to build up a profile that really is quite unacceptable."
Alliance Leader David Ford expressed revulsion at news of the shooting of the police officer.
He said: "My heart and the hearts of the whole community goes out to the family and colleagues of the police officer who has been so brutally murdered serving the people of Northern Ireland.
"I simply cannot comprehend the sheer evil of those involved in this attack. This shooting was cowardly and despicable.
"This murder must be roundly condemned by all right-thinking people. It is clear that dissident republicans are determined to attempt a sustained attack on peace and stability in Northern Ireland.
"All political representatives and the whole community united to condemn Saturday's attack. The public's resolve has been strengthened against these elements - everyone has spoken with one voice to say that peace is the only way forward.
"There must be calm at this time. Political representatives again need to show leadership.
"Anyone with any information on these attacks has a duty to contact the police immediately. The public, working closely with the police, can catch these individuals and end this violence."
Patrick Mercer MP, chairman of the Commons sub-committee on counter terrorism, said: "This is not unexpected. They have been expecting this escalation in activity for some time.
"The chief constable last week said the threat had risen and the fact that he asked for the help of special troops was a tactic that proves his fears.
"The terrorists want to damage the peace process - that is undoubted - but they are a small minority."
Since the start of 2008, dissident groups have mounted 18 gun and bomb attacks, with three carried out in the early months of 2009.
They had failed to claim lives during their latest campaign, until the soldiers from the 38 Engineer Regiment were gunned down outside the Massereene base on Saturday.
The news of tonight's killing emerged as tributes were still being paid to Sapper Mark Quinsey, 23, from Birmingham, and Sapper Patrick Azimkar, 21, from Wood Green, north London.
Last week nationalists and republican politicians criticised Northern Ireland Chief Constable Sir Hugh Orde after it emerged he had brought covert troops to help with the surveillance of dissidents.
But in the aftermath of the Massereene murders in Antrim, politicians showed a united front at Stormont today in opposition to those groups intent on violence.
Sinn Fein Assembly member for the Craigavon area John O'Dowd hit out at those behind the latest attack.
"This is an attack on the peace process. It is wrong and it is counter productive and I would extend my condolences to the dead man's family at this time," he said.
"As with what happened in Antrim over the weekend we condemn it. Whoever carried out this shooting was not doing so to advance Irish republican or democratic goals. They have no strategy to deliver a United Ireland.
"This is a time for strong political leadership and cool heads. It is a time for all political parties and the two governments to recommit to the principles which have underpinned the peace process and delivered the stability of recent years."
SDLP Upper Bann MLA Dolores Kelly condemned the shooting of the PSNI officer.
She said: "We are staring into the abyss. There is little point appealing to the people who planned and did this, but all of us have to realise we are on the brink of something absolutely awful.
"All of us have to get together to pull ourselves back from the brink. A tiny handful of people with nothing to say and nothing to offer cannot be allowed to destroy so much."
The leader of the loyalist Progressive Unionist Party Dawn Purvis MLA said her thoughts were with the officer's family.
She echoed comments she made after the weekend Real IRA attack: "The message remains the same, the perpetrators will not be allowed to damage our peace process. I appeal for calm and for people to assist the police by providing them with the information that can take these people off the streets and before the courts."
Prime Minister Gordon Brown was in Northern Ireland yesterday to meet security chiefs and political leaders.
He said those behind the Massereene attack would not threaten the political progress that had been made over the last decade.
The DUP and Sinn Fein are the largest parties at Stormont and they spearhead the power-sharing government in place in Northern Ireland.
Yesterday First Minister and DUP leader Peter Robinson told a packed Northern Ireland Assembly that the Massereene attack was a throwback to years of violence he said would never return, while Sinn Fein President Gerry Adams said his party would stand up to anyone who tried to undermine the political process.
Mr Robinson and deputy First Minister Martin McGuinness of Sinn Fein are due to fly to the United States for a series of meetings with business leaders and to hold talks with President Barack Obama.
Yesterday evening's events represented a further challenge to the political system, with dissidents intent on destabilising the Stormont administration.
Ulster Unionist Party (UUP) deputy leader Danny Kennedy branded the latest shooting as wicked and murderous and blamed dissident republicans.
"These terrorists seem totally incapable of understanding that they are flying in the face of the overwhelming will of the people in Northern Ireland, Great Britain and the Republic of Ireland, who want peace and political stability," he said.
"They are flying in the face of the unified opposition expressed to them in the Assembly and in the media today.
"And they will discover that the political parties and people of Northern Ireland are determined in their collective resolve to withstand any and every assault on the democratic institutions that have been in place for the last decade.
"We will not allow ourselves to be dragged back to a darker, bloodier world."
He appealed for anyone with information to help the police.
Police carrying machine guns were this morning guarding the scene of the shooting, which happened close to a school.
Councillors and local people were tonight gathering on the residential street as police helicopters hovered above.
A spokesman for the Police Service of Northern Ireland said police were called to Lismore Manor, Craigavon, at around 9.45pm, following a request for assistance from a member of the public.
He said: "Two police vehicles arrived in the area. Both officers alighted from the vehicles. It appears gunshots were fired at them. One officer was struck by gunfire and subsequently he has died from his wounds."
He said a major investigation was now under way and police are appealing for anyone with information to call Lurgan police on 0845 600 8000 or call Crimestoppers anonymously on 0800 555 111.