The burglars struck half an hour after midnight, just hours after a party to mark the tenth anniversary of the firm's relocation to the Norcote salerooms.
Using a vehicle to gain access to the premises, they tore off the saleroom doors with a 4x4 and took around 50 objects. Although the building was alarmed and wired to the local police, the thieves made away with their haul before the police arrived on the scene.
Detectives from the Gloucestershire Constabulary are now examining CCTV, and a list of the stolen items is being circulated.
Auctioneer Philip Allwood said: "Moore Allen had taken every necessary precaution to deter burglars, and this raid was obviously carried out by a well-organised and determined gang. The antiques were insured on behalf of our vendors."
Among the more distinctive items stolen were inscribed pieces of 18th and 19th century silver: a Victorian silver soufflé dish inscribed To the officers, 3rd Dragoon Guards, a Victorian silver desk standish inscribed To Joyce from Jock and Ann, and a George II silver sugar caster inscribed Presented to the Officers' Mess Scots Greys by Captain T Jessop March 1878.
Among the highest value items stolen were a George II Irish mahogany silver table, which carried an estimate of £7000-10,000, and a George II Irish giltwood side table, which carried an estimate of £8000-12,000.
Mr Allwood thanked his staff for their 'Dunkirk spirit' that saw a depleted sale proceed on Friday, May 29.
Anyone with any information regarding the raid should contact Gloucestershire Constabulary on 0845 090 1234 or Crimestoppers anonymously on 0800 111 555.