A witness said he heard a young woman screaming during a stabbing on a busy street in a daylight attack.
The man, who did not want to be identified, said he saw two men with a blade on Caledonian Road in Islington, north London.
“Two of them attacked one and basically they got stabbed pretty badly,” he added.
“I thought the woman was just screaming I didn’t realise she was hurt.”
The victim, in her 20s, is believed to have been stabbed in the chest.
Part of the street was in lockdown with cordons in place and traffic diverted following the attack at around 10.20am on Saturday.
Piles of clothes could be seen lying on the ground outside a parade of shops and cafes.
There was a trail of what appeared to be blood splatters along the pavement.
A man, who has not been identified, has been arrested and is being held by Metropolitan Police officers on suspicion of attempted murder.
A London Ambulance Service spokesman said: “We were called at 10.16am, there were a number of calls to us reporting a stabbing incident.
“We sent a number of vehicles and our first ambulance arrived on the scene at 10.24am.”