A police investigation has been launched after a post office robbery on Hampstead High Street.

Police were called to Hampstead High Street yesterday afternoon after reports of a “cash in transit” robbery.

A witness described how a man ran out of the shop and, as he escaped, hit the security guard in the face who collapsed to the ground bleeding.

Mohammed Khan, who was posting a parcel at the time, said: “There was a scuffle and then I heard cries and shouts of ‘Stop him!’ and saw a man running away carrying a bag.

“I tried to trip him up and then chased him down the road with the security guard. The guard collapsed after he was struck and I ran as fast as I could half way down the road but I couldn’t keep up.”

He added: “I didn’t know if he had a weapon. When you have a gut feeling you don’t think, you just do it.”

The suspect, described by witnesses as a white man and wearing a hooded top, fled the scene with the post office cash box before police arrived.

According to a police spokesman, an ambulance was called and the security guard was taken to hospital to be treated for minor injuries.

The spokesperson said that police did not believe the suspect to have been carrying a weapon.

They are now investigating the incident and urge anyone with information to call 101.