Five men have been arrested in a series of dawn raids involving more than 500 police from four different forces.
A suspect is led away by armed police officers
The men will be questioned over attacks at more than 60 post offices and cash machines over a period of eight months.
Police from Cambs, Suffolk, Norfolk and Essex swooped on five addresses, four of them at travellers' sites.
A spokeswoman said: "They were arrested on suspicion of conspiring to commit armed robbery and burglary as well as possession of firearms."
Police said the five men arrested were aged 17, 19, 20, 21 and 23.
The spokeswoman said the 17-year-old was arrested at an address in Christchurch, Cambs; the 19-year-old at a travellers' site in Willingham, Cambs.
The 20-year-old and 23-year-old were held at a travellers' site in Kirkham's Lane, Wisbech, Cambs; and the 21-year-old at a travellers' site in Mildenhall, Suffolk.
Police squads including armed officers and specialist search teams and the operation was estimated to have cost about £500,000.
The attacks on post offices and cash machines have been carried out in Cambridgeshire, Norfolk, Suffolk and Essex.