Armed police officers are engaged in a stand-off involving three people inside a house after they received reports of a man with a gun.
Specialist officers and negotiators have been on the scene for more than 24 hours.
They were called to the address in Badger Road, Costessey, Norwich, at 9am on Friday by a concerned member of the public.
An area around the property has been cordoned off, a Norfolk Police spokeswoman said.
It is believed that two men and a woman are inside the house but police have not commented on whether any of them are being held hostage.
The police spokeswoman said: "In the interests of public safety, specialist trained officers have cordoned off an area around a property following a call from a member of the public.
"Police negotiators are also on scene.
"The safety of residents in the Badger Road area is our priority and we are hoping to resolve the incident as soon as possible."
She said officers are trying to determine whether there is a firearm inside the property.
Local reports say that a dark-haired woman emerged into the back garden and threw an item - believed to be a letter - into the garden next door.