The Metropolitan Police is bracing for a deadly crowd crush at the Notting Hill Carnival.
The annual celebration of Caribbean culture is expected to attract around two million people over the bank holiday weekend.
Sir Sadiq Khan, the Labour Mayor of London, has said he fears there would be a mass casualty incident at the event, which he said had become a "victim of its own success".
On the eve of the carnival, police commanders warned that they still had grave concerns about the possibility of a mass crush despite organisers' insistence that new safety measures had been put in place.
Commander Charmain Brenyah, the Met's spokesperson for the carnival, said the force could not afford to be complacent.
She said: "The Met fully supported the event organisers' decision to commission an independent review into crowd safety after the number of situations in recent years where crowd density has reached dangerous levels.
"It was welcome news that some additional funding was secured to allow the organisers to implement a number of the review's recommendations.
"But they have had limited time to do so, meaning despite everyone's best efforts, it's inevitable not all the risks will have been mitigated."
Matthew Phillip, chief executive of the Notting Hill Carnival, said this year's event was "better resourced than ever".
He said that the extra funding had allowed them to deploy more trained stewards on the ground as well as a "team of crowd experts" who would operate on the streets and in a control centre.
There will also be greater use of technology and CCTV to help with crowd management, including large screens around the area, which will help direct the public, Mr Phillip said.
Sir Sadiq admitted in June that the risk of a crowd crush at the event "made him frightened" after seeing "images of some of the crowds at some parts of the day".
Earlier this month, a new report found that the carnival should be ticketed to avoid a Hillsborough-style "catastrophic failure of public safety".
Susan Hall, a Conservative member of the London Assembly, who produced the report, said the event had in recent years only narrowly "avoided a mass crush on the scale of the Hillsborough disaster".
Ms Hall's proposals include requiring all attendees to pre-register, with tickets "linked to an individual's billing address", so that the Met could "link potential crimes to individuals" and subsequently prosecute attendees and ban them from future events.
She also called for a maximum capacity, sponsorship and ticket revenue to cover policing and costs, and a possible relocation to Hyde Park if the streets of Notting Hill could not be safely managed.
In response to Ms Hall's comments, chair of the festival, Ian Comfort, said: "We're strengthening every operational element of an already extensive, multi-agency event liaison team: more trained stewards, expanded CCTV coverage, better parade coordination, faster response systems and enhanced crowd management infrastructure."
Two people, young mother Cher Maximen and chef Mussie Imnetu, were killed at the event last year in separate attacks.
Ms Maximen, 32, was stabbed at the Carnival in front of her three-year-old daughter as she tried to save a man from being attacked on the Saturday.
Mr Imnetu, who worked under the likes of Gordon Ramsay, died from "catastrophic brain injuries" after he was found unconscious near the carnival in Queensway on bank holiday Monday.
Ahead of this year's event, the Met said it had arrested 100 people as part of an operation to ensure the safety of those in attendance.
Officers carried out "intelligence-led interventions" and 21 people were recalled to prison, and 11 firearms and more than 40 knives were seized, the Metropolitan Police said.
The Met said 266 people either have police bail or probation licence conditions not to attend the carnival.
Alongside the thousands of police officers deployed each day to ensure the safety of carnival-goers, there will be facial recognition technology, weapon screening, and specialist crowd control teams.
The Equality and Human Rights Commission (EHRC) said earlier this week the force's rules and safeguards over using the tool "fall short", and could have a "chilling effect" on individuals' rights when used.
Issuing a warning to those intent on criminality, Cmdr Brenyah said: "The vast majority [of people] come to have fun and enjoy themselves, to celebrate Caribbean culture, to dance, to eat and to go home with nothing but good memories.
"Regrettably, we know a minority come with less positive intentions, and in recent years this has played out in the form of serious violence, including three tragic incidents where lives have been taken.
"The actions of this minority are totally at odds with the values of those who care passionately about carnival and we acknowledge those, including the event organisers, who have stood up to condemn violence and serious criminality in the run up to this weekend."