Thousands of police officers will be deployed among the Calypso dancers and steel drum bands at the Notting Hill Carnival following intelligence suggesting gangs want to create trouble there.
This year's festival also will end early, before dark, as a precaution.
The force said Friday it has already arrested 40 people in raids ahead of the two-day event, which celebrates Caribbean culture and typically attracts about 1 million people.
Scotland Yard said there will be 5,500 officers on duty in London's Notting Hill on Sunday and 6,500 on Monday, the main day of the carnival.
Thousands of officers will be on patrol elsewhere in the capital, creating a combined force that will dwarf the 5,000 officers who were on duty for the wedding of Prince William and Kate Middleton earlier this year.
The numbers reflect a city — and police force — still on edge after four nights of rioting and looting this month left the capital reeling, amid questions whether the initial police response was adequate.
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The disorder, which spread to other cities across England, was blamed on gangs stirring up trouble and prompted soul-searching about deeper-rooted social issues in Britain.
Scotland Yard Commander Steve Rodhouse said intelligence suggests gangs want to come to the carnival "and create trouble."
He told reporters at a media briefing this week that security operations had been reviewed in light of the recent unrest to take into consideration that the carnival would be taking place "in unusual and exceptional circumstances."
It bills itself as Europe's largest street festival. It attracts hundreds of thousands of people to its parades, rows of jerk chicken vendors and thumping sound stages set up throughout the west London neighborhood.
Public drunkenness and disorder at the event usually prompt a few hundred arrests each year.
Unrest has typically broken out after dark. To address concerns from performers and local businesses, festival organizers decided to wrap up the event a few hours early this year, at 7 p.m.
Chris Boothman, one of the carnival's co-directors, stressed that organizers are not expecting "anything out of the ordinary" and that past festival goers know there's nothing to fear.
He conceded that he could understand why recent riots in London might concern the uninitiated. To them, he said, "our message would be if you're planning to come, come early, enjoy yourself and get home safe."
Boothman said carnival organizers have no problem with the police reinforcements — as long as their presence doesn't change the flavor of the day.
"Our problem will be if the officers give the impression the police are flooding the carnival," he said.
Rodhouse shrugged off the suggestion that reinforced policing in Notting Hill will leave the rest of London without a strong policing presence, saying: "This is not the case."
In addition to the officers in Notting Hill itself, Scotland Yard said 4,000 ones will be on duty across London to complement the thousands of officers typically on duty on a normal weekend.
"To those who want to come to corrupt this magnificent event you are not welcome. Please do not come," Rodhouse said, adding that the force "will do everything in its power to make it as hard as possible for you."
Launched in 1964 with a few Trinidadian steel bands, the carnival has grown into a major street event that lures partygoers from all over the world.