Several people gathered at the scene in Market Street, Watford A man is staging a stand-off with armed police at a bank in Hertfordshire after threatening staff.
Police were called to the Co-op Bank in Market Street, Watford, at about 1020 BST.
Officers cordoned off a large area of the centre of the town and redirected traffic away from the area.
Police said the man was acting alone and no members of the public were at risk. An eyewitnesses spoke of police with guns.
Early reports suggested the man had a bomb strapped to his leg, but this has not been confirmed by police.
The owner of a tailoring business in Market Street said three police marksmen were aiming their weapons at the bank.
He said: “There are three of them and they are about 30 feet (10m) away from the bank on the opposite side of the road.
“There’s a lot of police around, pulling people back from the bank, and a few helicopters flying overhead.
“People are saying that the man in the bank has got a bomb strapped to himself and, as far as we know, all the hostages have been set free.
“A lady came out of the bank this morning and she was very distressed. I don’t know if she was a staff member, I think she might have been a customer.”
He said local people and business owners in the street had been told by police to stay indoors until the incident was resolved.
He added: “The police now are just telling people to get further and further away.
“They’ve told people to stay inside because, obviously, if he has got a bomb, they don’t know the extent of it or how big it could be.”
James Glisson, who was in a nearby mobile phone shop, said he had seen “military-looking” police officers and dogs passing by, but he could not see if any staff were still inside the bank.
Another eyewitness close to the scene told the BBC that police were using megaphones to speak to the people inside the bank.
He said that police officers nearby said it may be “a robbery that had gone wrong”.
Market Street is in the centre of Watford The police spokeswoman said: “We fully understand people will concerned by what is happened but would ask members of the public not to call the police.”
Hertfordshire Police said they would be updating details of the incident on Facebook and Twitter.
A Hertfordshire police spokeswoman said: “We have now been able to establish that this individual is on his own and no members of the public are at direct risk.
“Clearly, our objective is to resolve this situation as peaceably as possible and thank the public locally for their co-operation and understanding is a fast moving and complex situation.
“Can we ask members of the public to continue their cooperation with officers and listen to any instructions for their own safety.”
In a statement the Co-Op Bank said it was assisting police in managing the situation.
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