A policeman has been killed in County Armagh as he responded to a call from a "vulnerable" member of the public.
The officer was "gunned down" on Monday night as he got out of a car at Lismore Manor, in Craigavon, police said.
No-one has admitted to killing the officer, who is yet to be named. Police chief Sir Hugh Orde called it a "sad day" for Northern Ireland's force.
Politicians condemned the shooting, which happened two days after two soldiers were murdered in Antrim.
The soldiers were shot dead outside an army base on Saturday in an attack the Real IRA has said it was behind.
Politicians from all parties condemned Monday's killing, which Northern Ireland Secretary Shaun Woodward insisted would not damage the peace process.
It is the first killing of a serving police officer in the province for more than a decade and happened on a predominantly Catholic estate.
Sir Hugh, the chief constable of the Police Service of Northern Ireland, said it was too early to identify the killers.
He said the officer, understood to be an experienced officer, was with a colleague responding to a call from a "vulnerable person" when he was shot at about 2145 GMT.
"We are used to being attacked, but we will not step back," Sir Hugh said.
"This will not put off me or my officers delivering the service we do to the communities we are paid to protect."
Asked if the killing was linked to the soldiers' shooting at the weekend, 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."
Mr Woodward said: "It may be that this small number of criminals do regrettably have the capacity, at the moment, to take away life, but what they will never have is the capacity to take away from people the peace process and the political progress that's been made.
"My job and every politician's job in Northern Ireland is to give the people of Northern Ireland what they want, which is freedom, the rule of law and democracy - and we will do that."
First Minister Peter Robinson described the latest killing as "an evil deed".
"I am sickened at the attempts by terrorists to destabilise Northern Ireland," he said.
"Those responsible for this murderous act will not be allowed to drag our province back to the past."
DUP assembly member David Simpson described the attack as an "outrage" and said those behind it were "vermin".
"What we have seen over the last 36 hours is a deliberate and sustained effort by terrorist murderers to try to drag Northern Ireland back to the worst days of Ulster's past," he said.
Alex Maskey, a Sinn Fein Policing Board member, said that the murder was "yet another awful tragedy".
"I would like to send our condolences to [the officer's] family and express our disgust and anger that this has happened tonight again, after the weekend."
UUP Deputy Leader Danny Kennedy said: "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."
SDLP Upper Bann assembly member Dolores Kelly warned Northern Ireland was on the "brink".
"We are staring into the abyss," she said.
"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."
Alliance Party leader David Ford said the murder "must be condemned by all right-thinking people".
"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," he said.
"There must be calm at this time. Political representatives again need to show leadership."
In the weekend attack Sappers Mark Quinsey, 23, from Birmingham and Patrick Azimkar, 21, from London, were shot dead at Massereene Army base, Antrim.
The soldiers were killed as they accepted a pizza delivery at about 2120 GMT on Saturday.
Four other people, including two pizza delivery men - Anthony Watson, 19, from Antrim, and a Polish man in his 30s - were injured in the attack.