Nineteen people, including a boy, are being quizzed over a 90-minute daytime brawl in London's Trafalgar Square during which three men were stabbed.
Knives, hockey sticks and other weapons were used on Thursday as tourists and commuters looked on, police said.
A man aged 19 was stabbed in the head, arm and back but he has since been discharged from hospital.
A 17-year-old stabbed in the chest is in a stable condition, as is a 19-year-old wounded in the neck, back and leg.
Eyewitnesses said young men were lashing out on the pedestrianised area around Nelson's Column and also on adjoining streets between 1600 and 1730 GMT.
Officers are studying recordings from several CCTV and traffic cameras located around the square.
"This appears to have been a spontaneous incident and our officers were on scene extremely quickly," said Supt Simon Ovens of the Metropolitan Police.
It would "take time" to question the 19 suspects "and establish a clear picture of exactly what happened", he added.