A female security worker was robbed as she made a cash delivery inside an Accrington bank.
The incident happened at 11am on Friday in HSBC on Blackburn Road.
The woman, who worked for G4S security, was making the delivery when was confronted by a man wearing a hooded top who pushed her with force before snatching the money box.
He escaped on foot in the direction of Birch Street with a significant amount of cash.
No-one was injured during the incident.
Detective Inspector Mark Vaughton is appealing for witnesses following the incident.
He said: "A significant investigation has now been launched with officers conducting forensic examinations at the crime scene and carrying out enquiries with local residents and businesses in the area.
"We are also checking CCTV to try to trace the movements of the offender but I am appealing for anyone with any information to come forward. In particular, we are keen to hear from anyone who may have seen a man matching his description approaching or leaving the area.
"Finally, if you spotted anything suspicious in the Blackburn Road area either last night or in the days leading up to the robbery - perhaps someone watching the building - then we want to hear from you."
The lone offender is believed to be white, aged in his early twenties of medium build.
He was wearing dark clothing with a hooded top with dark track suit bottoms, with the hood pulled up over his face. He was also carrying a black holdall.
Anyone with any information should call Lancashire Police on 0845 1 253545 or Crimestoppers anonymously on 0800 555 111.