As Britain prepares to stage its first state funeral in nearly six decades, security officials are planning what is expected to be the "biggest policing and protective operation" in UK history.
Police are dusting off long-standing plans to keep safe attendees -- including global political leaders and royalty -- at Queen Elizabeth II's historic funeral, as well as the millions predicted to flock to London around the time of the event.
No date has been announced for the late queen's state funeral, which is the first since the service in 1965 for Winston Churchill, Britain's prime minister during World War II.
But it is expected to be held in Westminster Abbey in the heart of the capital on September 19.
Prior to that, her body is set to lie in state for four days in Westminster Hall, the oldest building on the UK Parliamentary estate, following a ceremonial procession through the streets of London.
The pomp-laden events, in particular the funeral, will require an array of elaborate security measures.
Hundreds of world leaders and dignitaries as well as millions of people are expected to descend on London.
They include US president Joe Biden, French president Emmanuel Macron, European Commission president Ursula von der Leyen, and leaders from countries where the British monarch is head of state or which are members of the 56-nation Commonwealth.
Japan's emperor Naruhito will be among royalty from around the world set to attend the funeral, according to reports, in his first overseas trip since he ascended the Chrysanthemum throne in 2019 following his father's abdication.
"This will be probably the biggest policing and protective operation the UK has ever mounted," Nick Aldworth, former Counter Terrorism Policing national coordinator, told The Independent outlet.
"It just takes one car, one person to do something abhorrent and not only have you disrupted a constitutional event, people will be injured and killed."
Aldworth noted the ceremonial events would occur in a "very different threat world" compared to previous royal funerals, such as the Queen Mother's in 2002 and princess Diana's five years earlier.
Britain has been hit by numerous terror attacks in the last decade, including a spate of atrocities in London, Manchester and other cities by jihadist extremists.
The current national threat level, set by the country's MI5 domestic security service and designed to give a broad indication of the likelihood of a terrorist attack, is "substantial".
That is in the middle of a five-tier ranking system rising from "low" to "critical".
London's Metropolitan police said Friday it had already begun deploying "well-rehearsed" plans for the official 10-day national mourning period, which will culminate in the landmark service for the country's longest-serving monarch.
"We will be keeping people safe with highly visible patrols across London," the force said.
"The public will see additional officers posted outside key locations, including transport hubs, royal parks and outside royal residences in London."
The Met is coordinating its policing blueprint with the British Transport Police, which is responsible for the national transport network, and City of London Police, which secures the so-called "Square Mile" financial district.
The Met's deputy assistant commissioner Stuart Cundy said the "comprehensive policing plan" would be most visible in and around Westminster, which is home to parliament, the abbey and Buckingham Palace.
British police have experience of handling major events, including last year in Glasgow, when hundreds of world leaders attended the UN climate summit COP26.
In a sign of the days of disruption to come in the heart of the capital, numerous roads were closed Saturday for events related to the proclamation officially kickstarting the reign of the new king, Charles III.
Officers were also increasingly visible on the streets.
"A great number of police officers will be on duty during this period, to ensure the safety of those visiting London and to deter any potential criminality," Cundy added.
But fears have been raised over the Met's ability to cope with the huge crowds expected in the run-up to the funeral, with three Premier League football matches currently scheduled for next weekend in the capital, and other games elsewhere.
UK media reported a potential shortage of police could see fixtures called off in London and beyond, as officers are set to be redeployed en masse to the capital.
All football matches were cancelled this weekend as a mark of respect to the queen, though some other sporting events are going ahead.