A man in his 30s has been arrested after a number of people were taken hostage at a bank near London's Heathrow Airport, Sky News reported.

Police said shortly before 4 p.m. local time, officers received reports that a man with a firearm had entered a Barclays in Ashford, west London.

Detectives said a number of people were inside the bank at the time and were prevented from leaving.

"This was a fast-moving incident and police were on the scene within minutes," said Superintendent Duncan Greenhalgh of Surrey Police. "Trained hostage negotiators made contact with an individual inside the bank. Just after 7pm, a man came out of the bank and was contained by armed police."

Greenhalgh said a man in his 30s had been arrested and was in police custody, adding that all the hostages were being assessed by medical teams and that there were no serious injuries.

"This was obviously a very traumatic experience for those members of the public and bank staff caught up in it," he said.

An apparent eyewitness described to Sky News the moment the gunman entered the bank.

Lianne Ralph said the man, who was wearing overalls, was armed with a rifle and told everyone to get on the floor. She said the man then started handing out overalls and solvent to staff so they could spray the windows with black paint.

It is believed this was so police could not see what was happening inside.

Ralph told Sky News that the gunman said anyone with a heart condition should tell him. Ralph said she was having an anxiety attack -- and he then let her go.

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