At least £19,000 was stolen from vulnerable people in Epping Forest last month by thieves posing as police officers or bank staff.
A total of 640 households in Essex were contacted by criminals targeting the elderly with a cash card scam.
Thieves telephoned homes claiming to be police officers or from a bank in order to secure cards or account details and steal money.
A minimum of £350,000 has been stolen in the county since January last year.
Police say victims should not to be embarrassed about reporting the crime and are reminded families to let all elderly and vulnerable friends and family know officers will never ask for personal information over the phone.
Dective Sergeant Neil Hudson said: "These are despicable crimes being carried by thieves posing as police fraud officers, the very people they ought to be able to turn to and trust in exactly this sort of crime.
"Instead, they callously prey on the elderly and vulnerable and have no thought for how their victims might suffer from the loss of money that might have been saved over many years."