Three balaclava-clad men burst into the Post Office in the Spar in Lytham Road, Freckleton, demanding cash this morning, police said.
After threatening a female member of staff, who suffered minor injuries, the trio stole an unspecified amount of cash, and cigarettes, a spokesman said.
They escaped in a black car, and are now being hunted by officers, who have appealed for witnesses to come forward.
The robbery happened at around 11.30am and the men were also dressed in black.
One man, who witnessed the robbery's aftermath, said: "I think they have beaten somebody up and gone to the owner with some sort of arms.
"There was about 10 police cars here before, now there's about four or five.
"There's a couple of police officers on the door and it's all taped off."
A spokeswoman for the North West Ambulance Service (NWAS) said: "We received a call at 12.07pm to reports of an assault.
"An ambulance was sent to the scene but it does not look like we have taken anybody to hospital."
Nobody at the store was immediately available for a comment.
"An investigation is underway and enquiries are on-going," police said in a statement.
"Anyone who witnessed the incident or has any information can contact police on 101, quoting log reference 0551."
Freckleton’s Post Office used to be in Preston Old Road. It moved to its current site following a consultation in 2014.
More to follow