The incident unfolded at Basepoint Business Centre in Andersons Road, where armed police are on the scene.
Two eyewitnesses described a chaotic and frightening scene as the man confronted a delivery driver and later threatened others.
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Richard, 55, who witnessed the events, said: “We got chucked out. We’re in the apartment sort of over there, which is where Angie works, and we just heard someone scrapping in the car park and he was basically fighting with the delivery driver from EVRI.
“We run to the gate and he was literally battering the hell out of his van.
“This guy literally sped off and then he stood in front of our gates yelling, angry with the whole world, with a knife in his hand.
“I’d recognize him if I saw him again, he looked like he was crazy.”
The man was described as wearing all black and having a beard.
Richard said the knife appeared to be a steak knife, and that the man was waving it around openly.
Angie Calleya, who called 999, said: “I saw him and I said are you okay?
“I spotted the knife and I thought, right, calm him down, calm him down.
“I just picked the dog up because he’s doing his and I just called the police.
“This lot were here within minutes.”
Richard added: “I reckon somebody from one of the garages chased him because he’s gone up.
“He was just shouting ‘I’m going to get you all’ and he was just yelling about all sorts.
“His arms were all in the air and it was just bizarre, really.
“He’d got nothing on him, he’d got no bag or anything.”
Angie described the man as wearing a black puffer coat and black trousers.
She said she did not recognise him and did not believe he was one of the homeless people who are often in the area.
She also noted how quickly police arrived after her call, suggesting officers may already have been in the area.
Hampshire police have been approached for comment.