Armed police have descended on a park in Didcot following reports of a man wielding 'massive knives'.
Nearby schools and other buildings have been put on lockdown in the Oxfordshire town as police corner the man.
A member of staff at a nearby care home said the person appeared to have "serious machete-type knives" with him.
She told the Oxford Mail: "He's just sat there - I think there's communication going on with police.
"It's just one man but he's got some serious machete-type knives and he looks very unwell.
"We've now got armed police everywhere and we're on complete lockdown - we're not allowed outside."
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Witnesses claim swarms of police arrived in the area shortly after reports the man had pulled out 'two knives' at a nearby Sainsbury's at 1pm.
One shopper claimed the man, who was wearing a purple bandana, tried walking out with a bottle of alcohol without paying but was confronted by a security guard.
The man, who did not want to be named, told the Oxford Mail: "The guy started to shout and tell him [the guard] to 'get out of his way', and then he started to count down from three.
"As he got to one, he put the bottle down on the floor quite hard and he pulled out a very large kitchen knife."
Terrified shoppers and the security guard began to back away.
The witness claimed the man then proceeded to wave a second knife before leaving the store.
Another claimed on social media: "A man was at Sainsbury's and held everyone in with 2 big knives over a bottle of alcohol. He's proceeded to the park."
Thames Valley Police are yet to confirm any details but are urging people to avoid the area.
André Dias De Silva, general manager of the Didcot Civic Hall, said 170 people were locked inside the building.
He described seeing around 60-70 officers outside while various areas in the town have been cordoned off.
A police spokesman said: "We are currently dealing with a fear for welfare incident in Smallbone Recreation Park in Didcot.
"No one has been injured. A cordon is in place. Please stay away from the area."
The incident that sparked the police operation is thought to have happened in the last hour.
One person tweeted earlier: "What’s going on in Didcot? Lots of vehicles with blues and twos heading into town. Is there something to worry about?"
Another added: "Thames Valley Police dealing with dangerous situation in britwell road, Didcot. Closed to no vehicles and pedestrians."
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