Two female police officers have died during an operation which led to the arrest of a man wanted by Greater Manchester Police.
Police said both unarmed officers suffered fatal injuries during a "routine operation" in Hattersley, Tameside.
A witness in Abbey Gardens reported hearing 13 gunshots and an explosion.
Dale Cregan, 29, has been arrested in connection with the officers' deaths and two previous murders.
He was arrested when he walked into Hyde police station a short time after the incident.
A bomb disposal team remain at the scene.
The witness, 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."
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 Bob 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."
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."
Mr Shewan said Mr Cregan had also been arrested on suspicion of two counts of murder in relation to the investigations into the murders of David Short and Mark Short.
David Short was killed in a gun and grenade attack in Clayton on 10 August.
His son, Mark Short, was killed in a pub shooting at the Cotton Tree Pub in Droylsden in May.