Officers and others remain under threat as police continue to question a man suspected of murdering two policewomen in a gun and grenade attack.
Sir Peter Fahy, Chief Constable of Greater Manchester Police (GMP), said he could not be sure all the explosive devices had been recovered from the area where Pcs Nicola Hughes and Fiona Bone were attacked on Tuesday.
He also warned of the threat from a long-standing feud between gangs in the area and said the force had issued Osman warnings - notices given to people under threat of being murdered or seriously injured - to "a large number of individuals".
Sir Peter said: "We are not confident that we have recovered all the grenades, we don't know for certain, so we've made it clear to our officers that the threat is still there.
"I would want that to be the message, this has been a long-standing criminal feud between different outfits in Manchester and in the Tameside area, and that threat is very much there."
Two men have been arrested and are being questioned over the deaths of the police constables - Dale Cregan, 29, who was one of the country's most wanted men, and a 28-year-old. The second man was detained in the Hattersley area on Wednesday on suspicion of conspiracy to murder, Sir Peter said.
Cregan, who was wanted for questioning over the deaths of father and son David and Mark Short, handed himself over to police shortly after the two officers were killed. It is understood that a desk officer kept him talking while an armed unit was scrambled to the scene, and he was arrested.
Pc Bone, whose family have flown over from the Isle of Man, died at the scene of the attack while Pc Hughes died a short time later in hospital.
The family of Pc Bone, who lived in Sale with her partner Clare and Clare's daughter Jessie, paid tribute to the "wonderful daughter, sister, partner and parent" in a statement. It added: "She was planning her marriage to Clare and this joy has been taken from us."
Meanwhile, tThe BBC has pulled the final episode of its drama Good Cop following the murders. The drama, starring former Hollyoaks and Luther actor Warren Brown, features scenes of a violent attack on a female police officer. A new transmission date will be confirmed in due course.