The crook, who is thought to have been wielding a knife, struck at the Royal Bank of Scotland in Dumbarton this morning around 9am.

It's believed he threatened staff with a blade as the branch was opening at around 9.30am and made off with around £10,000.

Four police cars and a helicopter descended on the scene within minutes of the alarm being raised.

Officers stood guard outside the brach, in the town's High Street, while colleagues interviewed a visibly distressed female staff member, believed to be in her 20s, inside.

Eye witnesses said they saw the culprit make off from the bank towards the River Leven and officers were seen to quickly converge on that area to conduct a search.

The helicopter circled overhead for at least half-an-hour in an attempt to locate the robber.

One eyewitness, who did not wish to be named, said: "I saw about half a dozen police run in, go round the back and check the bins.

"It's amazing to think someone could be so brazen as to rob a bank first thing in the morning and run off along a busy High Street."

The bank is currently closed, but staff remain inside. The nearest RBS branch is in Alexandria.