Boxing Day shoppers have fled in panic after two men became embroiled in a fight believed to have involved knives.
A video posted on Twitter shows people running out of the Intu shopping centre, which used to be known as The Glades, in Bromley, south London.
A man can be heard saying "everyone is running in Bromley you know".
Officers arrested a man on suspicion of affray and possessing an offensive weapon, and are hunting another man who is believed to have run off, possibly dropping a weapon.
A Scotland Yard spokeswoman said: "We were called at 3.45pm to reports of an injured man at the Glades shopping centre.
"Officers arrived at the scene and found a man believed to be in his 20s with a minor head injury.
"At this stage officers believe he sustained the injury as a result of an altercation. The injured man has been arrested for affray and possession of an offensive weapon. He currently remains in custody at a south London police station.
"Officers have recovered two knives and are seeking to trace a man who was seen running off, possibly discarding a weapon. Enquiries continue."
Keith Moon, 35, an app developer, was out shopping with his wife when he suddenly saw shoppers running through the centre.
He later saw a young man in handcuffs and bleeding from the head being led away by police, who were carrying a 2ft-long rusty machete believed to have been recovered from the scene.
He told the Press Association: "We could see people running towards the exit, it wasn't quite clear what it was at first. But then they seemed to be running in panic.
"My wife heard someone say that there was a gunman running around so we ran towards the exit.
"We got outside and there were a lot of people, some quite emotional."
After about 20 minutes Mr Moon and his wife walked around to another of the centre's entrances and walked in to find police officers leading away the man in handcuffs.
He said: "We walked back in and saw the police leading a young guy in handcuffs away. One of the policemen was holding a machete around 2ft long. It was really, really rusty - it didn't look clean.
"The young kid was bleeding from his head.
"We ventured further into the Glade and the shoe shop Aldo was cordoned off and you can see stuff has been knocked over and I think there was a little bit of blood."
Mr Moon described "chaotic" scenes as shoppers fled and said his wife feared it was a terror attack.
She said: "It was really chaotic, especially given recent events it was pretty scary. My wife was concerned it could be a terrorist attack. It was definitely a concern. And when you find out it is a guy with a machete and you don't know the reason why, that is scary."