Hundreds of shoppers, out to grab bargains in the UK's traditional Boxing Day sales, were seen fleeing as armed police stepped in to arrest the man.
Rumours that a gunman was on the loose quickly spread through Intu shopping centre in Bromley area of the city, yesterday sparking alarm.
A video posted on Twitter shows shoppers, including families, screaming and running out of the centre.
A man in his 20s, who suffered a minor head injury, was arrested.
He was detained on suspicion of affray and possession of an offensive weapon.
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"Officers attended the scene and found a boy with a minor head injury.
We believe he sustained it as a result of an altercation.
"The injured boyhas been arrested for affray and possession of an offensive weapon.
"Detectives are seeking to trace approximately six youths who ran off from the scene shortly after the incident."
Boxing Day is a holiday celebrated the day following Christmas Day, when servants and tradesmen would receive gifts, known as a "Christmas box" from their masters, employers or customers.