Two female police officers have died during a "routine" call-out which led to the arrest of a man wanted by Greater Manchester Police.
PC Nicola Hughes, 23, and PC Fiona Bone, 32, suffered fatal injuries in the incident in Mottram, Tameside.
Dale Cregan, 29, has been arrested in connection with the officers' deaths and two previous murders.
Greater Manchester Police Chief Constable Sir Peter Fahy said it was one of the force's "darkest days".
Mr Cregan was arrested when he walked into Hyde police station a short time after the incident.
A witness at the scene in Abbey Gardens reported hearing 13 gunshots and an explosion shortly before 11:00 BST.
Police said one of the officers died at the scene and the second was critically injured and died afterwards.
The scene has been cordoned off and there is a heavy police presence in the area, including a bomb disposal team.
A police helicopter is also on patrol overhead.
The witness who described the shots, a window cleaner who worked in the area, said the property the officers were called to had been unoccupied for some months.
Assistant Chief Constable Garry Shewan said it was "a tragic day" for Greater Manchester Police.
"A long and dark shadow has been cast across Greater Manchester and my thoughts are with the officers' families, friends and colleagues.
"We will be doing everything we can to support them over the coming days and weeks."
The BBC's North of England reporter Danny Savage said he had spoken to an eyewitness who said they saw a police car outside shortly after 10:15 BST.
'Bang, bang, bang'
"After that car had drawn up it seems that the two unarmed police officers inside the car went into the property which had stood empty for some months," he said.
"Whilst they were inside around a dozen shots were fired and there was an explosion."
He added: "There were gunshots fired, possibly a hand grenade as well - that appears to be the suggestions at the moment.
"Those police officers didn't come out of the house again, the police car was left outside with its lights flashing but empty. Emergency services and colleagues were soon here offering assistance."
Prime Minister David Cameron said the killing was "a shocking reminder of the debt we owe to those who put themselves in danger to keep us safe and secure".
Home Secretary Theresa May said: "This is a deeply shocking incident and a terrible reminder of the risks that police officers face every day to keep our communities safe."
Shadow home secretary Yvette Cooper said it was "deeply distressing news [and] a painful demonstration of how police officers put themselves in harm's way every day to protect the public".
Witness Warren Sheppard was up a ladder cleaning the windows of a house nearby when the shooting happened.
He said: "I heard about 10 shots quite close, bang, bang, bang, like that. I knew they were gunshots because sometimes there is clay pigeon shooting in the fields.
"I heard a big explosion about 10 seconds after the shots. I got off the ladder, walked round, saw an empty police car.
"A car went speeding past to the main road."
He said he went back to his van but could see the aftermath through a gap between two houses.
"It was like something on the movies," he said.
"It's just so sad for the family of the two police women.
"Both lost their lives just doing their jobs and my heart goes out to their families and I know people around here will feel the same."
He said the house where the incident took place was a council house which had been left empty.
He said there had been some recent activity with the windows covered in whitewash suggesting the house was being decorated.
A 27-year-old woman, who did not want to be named, said her ex-boyfriend witnessed the shootings.
She said he was walking back from the doctor's when he heard someone shout to him.
"Then someone has come outside the house... and shot two officers and then he threw a grenade in the garden," she said.
"He said the officers hit the floor.
"Cregan then got into a silver BMW and drove off but he must have then gone on foot shortly after because my friend saw him on Stringer Avenue, which is only down the road."
The police flag at Greater Manchester Police headquarters has been lowered to half mast.
President of the Association of Chief Police Officers Sir Hugh Orde said the deaths of the two officers were "deeply sad news for the police service".
"Whenever police officers and staff lose a colleague that loss is felt right across the police family," he said.
"The thoughts of everyone in policing are with colleagues in Greater Manchester Police, family and friends of the two officers at this time."
Police had been offering a £25,000 reward for information leading to Mr Cregan's arrest as he was wanted over attacks that left a father and son dead.
ACC Shewan said Mr Cregan had also been arrested on suspicion of two counts of murder in relation to the investigations into the deaths of David Short and Mark Short.
David Short was killed in a gun and grenade attack in Clayton on 10 August.
His son, Mark, was killed in a pub shooting at the Cotton Tree Pub in Droylsden in May.
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