Two masked thugs threatened a cash in transit security guard in Allerton at knifepoint before escaping.
At around 1.45pm today, officers were called to Springwood Post Office on Greenhill Road to reports of a cash box being robbed at knifepoint.
It is believed that two men with their faces covered made off in a dark coloured vehicle towards Melbreck Road.
Enquiries are ongoing to establish what was stolen and detectives appealed for information.
Residents reported seeing armed police, cordon and forensics in the Westwood Road/Melbreck Road area this afternoon.
Detective Inspector Jennie Beck said: "These are the early stages of our investigation, and we want to hear from anyone who was in the area and saw this incident, or saw a small black car making off.
"To threaten someone who is carrying out their daily work is despicable.
"Your dashcam, CCTV or mobile phone footage could be the key, so tell us what you know and we will take positive action."
You can pass information via @MerPolCC, 101 or CrimestoppersUK anonymously with reference 423.
