Greater Manchester Police has warned of the threat from a long-standing feud between two criminal families in the area and has issued Osman warnings - notices given to people under threat of being murdered or seriously injured - to "a large number of individuals".
Assistant Chief Constable, Garry Shewan, said the murders of PC Fiona Bone and PC Nicola Hughes were a "watershed" moment and that the force's message was "Enough is enough. This has to end."
He added that despite everything the force has done to tackle gun crime, "tragedies still do occur".
"Tuesday's events simply make us spur on to higher and greater efforts."
The BBC has pulled the final episode of its drama Good Cop tonight after the murders of policewomen Fiona Bone and Nicola Hughes.
The drama features scenes of a violent attack on a female police officer.
In a statement, the BBC said: "In light of news events, BBC1 has postponed the final episode of Good Cop due to transmit at 9pm tonight."
Replacing the drama will be a repeat of the comedy series Outnumbered, followed by an edition of Have I Got Old News For You.
A new transmission date for the Good Cop episode will be confirmed in due course.
Greater Manchester Police have told ITV Central they do not believe the weapon used in the murder of PCs Fiona Bone and Nicola Hughes was smuggled into the UK by Steven Greenoe.
Greenoe, a former US Marine who lived in Shrewsbury, was jailed for ten years in January for a major gun-smuggling operation from America into the UK on commercial transatlantic flights.
The GMP told ITV Central: "There is no evidence to suggest the guns came from him. It is not a line of inquiry."
Police have been given more time to question 29-year-old Dale Cregan on suspicion of the murders of PC Fiona Bone and PC Nicola Hughes.
A warrant of further detention was obtained on Wednesday night and is due to expire at 9am on Friday morning.
A 28-year-old man who was arrested on suspicion of conspiracy to commit murder yesterday also remains in police custody
Greater Manchester Police has been inundated with messages of support from the public since the deaths of PC Fiona Bone and PC Nicola Hughes.
- The force has received 33,500 messages to its online book of condolence.
- More than 21,000 messages have been posted on the force's Facebook page.
- More than 1.3m people have viewed the tribute to the officers on Facebook.
The gun used in the murder of two unarmed police officers in Manchester on Tuesday may have come from a collection of firearms smuggled to Britain from the United States, The Times reports.
Police are now investigating whether the weapon recovered from the scene of the murders of PCs Fiona Bone and Nicola Hughes was one trafficked by Steven Greenoe.
The cousin of PC Nicola Hughes, who was shot dead in Manchester on Tuesday whilst on a routine patrol, has said her death "hasn't sunk in yet".
24-year-old Naomi Walker also described her cousin as "amazing".
She told the Manchester Evening News: "Our mums were pregnant at the same time and I have known her all my life.
"She would do anything for anyone. She honestly could not do enough for you. She used to invite me round for tea and before we went out she’d invite me around to hers to get ready together.
"She had always wanted to be in the police. It was really hard for her to get in and she had to do lots of training. I’ll never forget when she got in. She was thrilled and was texting everyone. She had to do some crazy shifts but she loved it."