Greater Manchester Chief Constable Sir Peter Fahy said Dale Cregan is thought to have made a bogus burglary report then attacked the officers, opening fire with a gun and a grenade.
Sir Peter named the "brave" officers as Fiona Bone, 32, and Nicola Hughes, 23.
He said: "It would appear Cregan has deliberately done this in an act of cold-blooded murder." He said Cregan's motive for the attack was "impossible to fathom".
After the attack, the fugitive, already wanted in connection with separate gun and grenade attacks that killed a father and son, gave himself up at Hyde police station in Grreater Manchester.
Sir Peter paid tribute to Miss Hughes, describing her as a "chatterbox" and a "great bobby" who was "always smiling".
He said Miss Bone was a "calm, gentle woman", an "excellent bobby" and had been in the middle of planning her wedding.
The Chief Constable said it was "routine" to send unarmed officers to a burglary call.
"We believe that Dale Cregan was in a house in Abbey Gardens overnight, and at some point this morning has either himself has made a call or had somebody else make a call reporting a burglary," Sir Peter said.
"This particular address was not known to us. So as would be routine, two unarmed officers were sent to the scene.
"When they arrived, it appears that Cregan emerged into the road and killed these two officers. A firearm was used, a grenade was also used."
Officers involved in the hunt for Cregan were "shattered" by what had happened.
Sir Peter continued: "Clearly we are devastated today by the loss of two of our officers. This is one of the darkest days in the history of the Greater Manchester Police if not for the police service overall, because we have lost two deeply loved and valued colleagues, because they are part of our team, policing is very much a family.
"But also because of the huge efforts that officers had been making to arrest and detain Dale Cregan. Obviously the officers involved in that inquiry are shattered by this outcome."
Cregan, who only has one eye, had been the subject of a huge manhunt after the murders of David Short, 46, and his son Mark, 23. A £50,000 reward had been offered for information leading to his arrest.
David Short was killed at his home in Folkestone Road East, Clayton, Manchester, on August 10, while Mark was gunned down at the Cotton Tree pub in nearby Droylsden, on May 25.
Four men have already been charged in connection with Mark Short's murder and are due to enter pleas at Manchester Crown Court in November. Earlier this month a 33-year-old man also appeared at Manchester Crown Court charged with the gun and grenade murder of David Short.
Sir Peter, speaking at a press conference at GMP headquarters, said there was nothing in the burglary report to suggest that armed police would be required. But as PCs Bone and Hughes arrived, Cregan attacked.
He said he believed Cregan had been "protected by a criminal conspiracy to harbour him", adding that the force was "fully determined" to investigate that conspiracy and bring those involved "to book". He said he was not aware that Cregan had contacted police at any time during their manhunt or made any threats to police officers.
But he added that police had been looking at a range of scenarios including Cregan trying to kill other people as a result of the gangland feud he was involved in. And he said there was "concern" that he might target police officers.
Sir Peter said finding Cregan after Mr Short's murder had been "a top priority" and "a huge investigation" for the force, involving 50 armed raids to try to locate him. He said officers showed "great courage" going into dangerous and unexpected situations.
"These were two officers going about their normal duty. Like all officers they went to a variety of incidents not knowing what it was that they would face.
"Clearly the police service is not perfect, we know there have been a number of high-profile incidents, but below that, day in, day out, police officers go about their duty, go into dangerous situations, unexpected situations, and show great bravery, great courage and are with people at the very worst moments in their lives. This is exactly what these two officers were doing."
Sir Peter said his force believed "passionately" that police should remain unarmed, despite the tragedy. He said: "Clearly we are a police force that is routinely unarmed, although we have great expertise from armed officers in our support.
"We are passionate that the British style of policing is routinely unarmed policing. Sadly we know from the experience in America and other countries, that having armed officers certainly does not mean, sadly, that police officers do not end up getting shot."
Eyewitnesses said a hail of bullets was fired and a grenade was used during the attack in Hattersley, Greater Manchester, shortly before 11am. One of the officers died at the scene. The second was critically injured and died later.
Window cleaner Warren Shepherd said: "I just heard gunshots, bang, bang, bang - around ten of them, then a pause and a big explosion."
The scene of the attack was cordoned off today. A fleet of vans and ambulances were parked at the top of the road as a helicopter hovered overhead. Traffic was blocked from coming on to the estate up to half a mile away. A Royal Logistic Corps bomb disposal vehicle was inside the cordon.
A man wearing jeans and a white T-shirt left a floral tribute at the side of the cordon. A note written on the back of a school letter said: "To the families and work colleagues of the two brave officers that laid down their lives. Our thoughts and prayers go to them."
Prime Minister David Cameron said that the killing of two police officers was "a shocking reminder of the debt we owe to those who put themselves in danger to keep us safe and secure".
Chairman of the GMP Federation Ian Hanson called the deaths "the slaughter of the innocents".
"I'm going to look beyond the uniform here. What we've got are two young girls that went out this morning and they've got an absolute right to come home tonight to their loved ones. This is cold-blooded murder. It's the slaughter of the innocents. GMP is a family.
"I'm struggling to find the words to use to officers out there who've lost friends and colleagues. It's a dark day for GMP, it's a dark day for policing, it's a dark day for society."
Chairman of Greater Manchester Police Authority Paul Murphy said: "They exemplified the dedication that officers show, the bravery that officers show every single day. These were two fine police officers that went to what was considered to be routine, and lost their lives."
