Armed police were called to a flat in Preston after receiving information that an alleged attacker might have a gun.
Officers attended the scene in Fishergate Hill, Preston at around 6am on Friday, in response to an earlier report of a domestic assault.
They said they received information that the person they were looking for was inside the building and potentially might have access to a firearm.
Lancashire Police said specially trained armed officers were sent to the flat.
The road was closed and a cordon was put in place until the incident could be resolved.
A police spokesman said an arrest was made and a firearm was recovered at the scene.
The Evening Post understands the gun was not fired during the incident and there is no suggestion that it was used in the assault or that any threats were made.
A 20-year-old man was arrested on suspicion of section 39 assault and possession of a firearm. He is currently in police custody.
The arrest was made at around 6.15am and the road was reopened at around 6.40am. The incident caused delays to a number of buses.