A woman had part of her top lip bitten off by an unknown man at Notting Hill Carnival who told her he was “the devil”, police said.
The woman, who does not want to be identified, was on Portobello Road near Golborne Road when he leant forward and bit her.
She has been left with life-changing injuries and will need “extensive surgery”, an officer added.
The attack happened on the first day of carnival – 25 August – at 18:00 BST.
The suspect has been described as a black man with short black hair and of muscular build. He was wearing a black T-shirt.
Officers have appealed for anyone who may have witnessed what happened to come forward.
Det Ch Insp Hayoukane said: “This was a horrific incident on a woman who has suffered life-changing injuries, requiring extensive surgery.
“The man who did this is clearly a violent and dangerous individual and I am determined that we identify and apprehend him as soon as possible.”
More than 350 people were arrested across both days of the Notting Hill Carnival, the Met Police has said.
A Section 60 order, giving police additional stop and search powers, was enforced on Monday after reports of “incidents of violence”.
Thirty officers suffered minor injuries at the event, with 37 people held for assaults on police.
Cdr Dave Musker said he was “happy” with how Carnival went but “extremely disappointed” by the attacks.
“Officers put themselves on the frontline and should, under no circumstance be assaulted for protecting the public,” Cdr Musker, who was the Met’s lead officer for the event, said.
By 22:00 BST on Monday, 353 arrests had been made with 111 on Sunday and 242 on the second day of the event.
The majority – some 162 – were for drugs, while 34 were for possession of offensive weapons, 31 for public order offences and 10 for sexual attacks.
An additional 30 arrests related to the event were made by British Transport Police on London’s transport network.
A Section 60 order had been authorised for specific stations, lines, and trains on Monday “to protect the travelling public and prevent serious violence”, the force said.
More than one million people attended the festivities as record temperatures were recorded in west London.