The thieves attacked the van in Stockholm's Östermalm district at around 1.30pm on Friday, with witnesses suggesting that the robbers were carrying what appeared to be guns.
According to a spokesperson for Stockholm police, Albin Näverberg, who was interviewed by Swedish newswire TT, "at least two robbers with weapon-like objects" managed to steal a case full of money,
"I stood on the street and saw three masked men," a bystander told Sweden's Metro newspaper.
"They ran around a car and jumped into it to escape," the witness added, explaining that the vehicle had then crashed into another security van that was attempting to block the path of the robbers.
"I'm almost a hundred percent sure that one had a gun," the observer said.
Police told TT that officers were now searching for a small blue car believed to be the suspects' getaway vehicle.
It is understood no one was hurt in the raid.
On Friday afternoon no details had been released about where the money was being transported from.
The robbery follows a spate of attacks on banks and foreign exchange bureaux in the Swedish capital in recent months.
In the most high profile heist in October 2014, branches of Handelsbanken and Nordea were targeted by thieves along with an exchange kiosk in the same part of the city as the latest attack.