A bank robber ran straight into an off-duty policewoman as he fled with his haul into a busy shopping street.

The man demanded the money from staff at the Royal Bank of Scotland branch in Castle Square, Brighton, just before 4pm today.

But he was foiled when PC Andrea Leahy noticed red security dye, which is used to protect banknotes, dripping from his pocket as he tried to get away.

The man had gone into the central Brighton bank and demanded money, claiming he had a gun.

He threatened staff who handed over several hundred pounds in cash and walked out of the bank.

As he went to approach a bus PC Leahy spotted the tell-tale red dye.

It is thought the security mechanism on the money had been triggered shortly after he left the bank.

PC Leahy, who was walking to the bus stop with her partner, stopped the man.

Each held onto an arm to stop him from pulling out any weapons.

They kept him in Palace Place until armed police arrived. The man was searched but found to be unarmed.

PC Leahy's actions won her praise for her bravery and quick thinking.

Chief Inspector Laurence Taylor said: "Normally, a police officer would be protected with a vest, a baton, captor spray and a radio but being off duty PC Leahy had none of these things at her disposal.

"She acted incredibly bravely, tackling a potentially very dangerous individual who had just moments before indicated to bank staff that he was armed.

'Although it appears these were verbal threats, it does not take away the seriousness of the offence."

The arrest caused a stir in the busy street.

Passers-by, workmen, and shop staff gathered to watch the armed police surround the handcuffed man, while office workers peered from over-looking windows.

A 32-year-old onlooker said: "I was just walking down towards North Street. I came around the corner and there were armed police and a man was clutching what looked like a load of £50 notes covered in red dye.

"Everyone just stood around quite stunned. I missed the initial chase so I didn't see anything which happened before.

"He looked like a normal bloke.

"The armed police were right by the guy between the Royal Bank of Scotland and the newsagents at the bottom of North Street."

Staff at Cologne and Cotton, next door to the bank, heard a commotion outside their back room.

Natalie Hogben, 25, said: "I heard them while I was out the back.

"I could hear the police radio.

"I opened the back door and saw a policeman who looked at me to say, 'Go away'."

Chloe Waites, 20, said: "It was exciting seeing a gun on a Tuesday afternoon.

"We haven't seen anything like this before."

Detective Sergeant Helen Campbell, of Sussex Police, said: "A 45-year-old man has been arrested on suspicion of robbery."

The man was still being questioned by Brighton CID officers tonight.