It is the largest security operation in the history of the Metropolitan Police (Met).
Maintaining order during the funeral of Queen Elizabeth II will prove "extremely complex", confides an assistant commissioner of the Met.
With a device even more important than that set up for the 2012 Olympic Games or for the platinum jubilee weekend.
According to the British press, 2 million visitors are expected this Monday, September 19 in the streets of London for this state funeral.
A crowd that will have to be supervised and monitored, especially around Westminster and Buckingham Palace.
The police will at the same time have to manage the security of the 2000 personalities invited to the ceremonies at Westminster Abbey, including 500 foreign leaders.
London had not welcomed so many VIPs since Winston Churchill's funeral 57 years ago.
“The biggest risk will be from lone wolves or knife attacks”
If Scotland Yard refuses to confirm the figure, there is talk of 20,000 officers deployed, with some 2,000 police officers from Scotland and Northern Ireland to lend a hand to the London teams.
Special counter-terrorism units are mobilized and work behind the scenes with MI5, the intelligence service responsible for UK internal security.
In the heart of the capital, neighborhoods will be squared, 36 km of barriers are installed to channel the crowd.
Around Westminster, snipers are placed on rooftops, manhole covers are sealed, lampposts examined and new surveillance cameras have been activated.
The police fear terrorist attacks.
“The biggest risk will be from lone wolves or stabbings by individuals, which are harder to predict,” said former royalty protection official Dai Davies.
On Friday, a man lunged at two police officers near Leicester Square, seriously injuring one of them.
An attack that would not be linked to terrorism or the death of the queen, according to the police, but which encourages us to be extra careful.
