The driver and his mate stood by helpless as the raiders unloaded and forced open cash boxes before escaping with the money.
Police say the final haul will not be known until the boxes have been checked. Securicor would not confirm the amount. but said it was offering a pounds 100,000 reward.
The raid happened just before 1pm, when the van left its depot in Powder Mill Lane, Dartford, Kent. It stopped a few minutes later at shops where the security staff were threatened at a newsagents while buying papers.
The van was driven to a remote country spot at Craylands Lane, in the nearby village of Swanscombe, where the robbers started unloading the cash.
The cash boxes contained varying amounts, and the total - estimated to be between pounds 3-pounds 6m - cannot be added up until the opened boxes are checked against invoices.
Detective Chief Inspector Alan Gimes of Kent Police told how the guards were then kidnapped and taken on a nightmare three-mile journey.
"There is a suspicion that the raiders possibly know the families of the two guards," said the officer. They told the guards that unless they co-operated there would be trouble.
"One of the guards had something sharp-pointed pushed against his back. And the other man was hit round the head.
"They both suffered shock, but were not badly hurt."
A spokesman for Securicor said: "An amount of cash was stolen during the attack and one of our crew members suffered minor injuries.
"Securicor is doing everything possible to assist the police with their investigations and we hope that someone will come forward with vital information."