A man was stabbed and another suffered head injuries in what one witness described as a "mass brawl" at a shopping centre.
Armed police responded to reports of a fight at Westfield in Stratford, east London, at around 6.15pm on Friday.
One man was taken to hospital by ambulance with a stab wound and his condition is unclear, while a second man was found with head injuries that are not thought to be serious.
A man was arrested nearby on suspicion of grievous bodily harm and possession of an offensive weapon, the Metropolitan Police said.
A spokeswoman said: "Inquiries into the circumstances continue."
BBC reporter Justin Dealey tweeted: "Thought I would come to Westfield Stratford for a change. Within minutes I've seen a mass brawl in the shopping centre. Classy gaff."
Another witness, who gave his name as John, said he was on his way to meet his children when he saw the aftermath of the fight.
He said: "There was a crowd of people looking at a youth being arrested, a man punched him. The mood was aggressive and voyeuristic. There were lots of kids about, the youth was punched during his arrest by one of the crowd.
"I walked further into Westfield and spotted the injured person. The photo is the victim, outside the Deichmann shop, ground floor. The arrest was near Greggs. There were armed police present, given the current climate I understand that."