Police blocked off part of Twickenham town centre after two robbers attacked a guard delivering money to a bank.
Officers erected a cordon around HSBC, in London Road, following the raid at lunchtime today.
The robbers punched the cash-in-transit guard in the head before snatching a money box he was carrying.
Acting Chief Inspector Hannah Wheeler, of Richmond police, said the two robbers fled on foot.
She said: “It wasn’t an armed robbery, a cash box was being delivered and two males made off with it. They punched the delivery man in the head and made off.”
Police believe the men may have had a getaway vehicle parked nearby.
Gurnam Sokal, the manager of Fone Gadgets said he often saw the guard visiting the bank.
He said: “He came out of HSBC looking for his glasses. He looked a bit baffled, but I didn’t realise what was going on.
“The police were there within seconds and they helped him.”
A spokesman for G4S said the guard was bruised but otherwise unhurt.
Employees at estate agents Chase Buchanan said they had not seen anything, but had heard an alarm going off across the road.
Lourdes Ebritt, manager at the Money Shop, said: “I didn’t see what happened but I noticed the police had cordoned off the area and there was a G4S driver talking to them.
He looked quite young, about 30, and he must have been explaining what had happened.”
The police took away the cordon at 1pm, but continued to speak to staff inside the bank.
A spokeswoman for HSBC would not reveal how much money the robbers got away with, but said the bank would hand its CCTV footage to police to help with their inquiries.
She said: “All staff are absolutely fine, we are helping police with their investigations.”
Police said the robbers were both black, wore woolly hats and used scarves to cover their faces.
They were wearing dark clothing, were of slight build and were believed to be in their mid-20s.
Witnesses said they saw a car speed away from the scene and officers have been working to find out if it was connected to the robbery, police said.
Anyone with information should call Crimestoppers on 0800 555 111.