An AK47, a World War II machine gun, and a rocket launcher were seized by police after a street fight broke out in Bromley.
Two men were arrested after police were called to reports of a fight involving a group of around 10 men, in East Street, Bromley, at 4.30am on Sunday.
As officers rushed to the scene, CCTV operators spotted one man carrying what looked like an assault rifle.
The group fighting had dispersed upon the arrival of officers at the scene, but investigators managed to trace the man with the suspected gun to a nearby address in Market Square using CCTV.
Specialist firearms officers entered the address and five men were arrested and three weapons seized.
The weapons were an AK47 style assault rifle, a WWII machine gun, and a rocket launcher.
Following examination by firearm specialists it was established that the weapons were incapable of firing and had been decommissioned.
A 32-year-old and a 41-year-old were taken into custody at a south London police station on suspicion of firearms offences. They remain in custody at this time.
Although the weapons were not capable of firing, the presence of firearms in a public place causes panic and fear of violence, a Metropolitan Police Service statement said.
Detectives from Bromley Borough's CID are investigating and enquiries continue.