The day of the Queen Elizabeth II's funeral marks the climax of what is being regarded as the biggest security operation the UK has ever seen.
Huge crowds, royalty and a long list of world leaders as well as other dignitaries will all need to be kept safe as part of the mammoth job facing thousands of police officers.
Metropolitan (Met) Police Deputy Assistant Commissioner Stuart Cundy said "nothing can compare" to the "hugely complex" task, describing it as the "largest policing operation" in the Met's history.
The scale surpasses the operation for the Platinum Jubilee weekend and the London 2012 Olympics, which saw up to 10,000 police officers on duty per day.
It is also the largest global protection operation dealt with by the force, with world leaders, dignitaries and other VIPs attending the state funeral.
Former counter-terrorism police chief Nick Aldworth said it is "probably the biggest operation that we're likely to mount in the UK."
It comes at a time when the country's terrorism threat level stands at "substantial," meaning an attack is "likely."
Police and security services will be alert to the prospect of knife attacks, bombs being detonated, and all other possible terror threats or incidents.
So-called lone actor terrorism, in particular knife attacks, are now considered the main threat. But police guarding the new king and senior royals also have to consider the risks posed by people who are fixated with those in the public eye.
Members of the public have been urged to report any suspicious behaviour, with security experts describing potential terrorists among the crowds as people who would seem "blatantly out of place" and uninterested in ceremonial events.
The risk of a threat is "fairly high" particularly when the new king, Charles III, comes into contact with crowds, according to a royal security expert.
Richard Aitch, director of operations for firm Mobius International, said it becomes "very difficult" for personal protection officers when the monarch is so close to a "group of unknowns" like crowds and they will be carrying out a split second process of "personal vetting" to root out suspicious activity.
He told the PA news agency: "As they're walking along the route, they'll be looking at hands and eyes. They'll be looking at the demeanour of the crowd. And what they do in their mind is they'll conduct the process of personal vetting. They will be doing their own vetting of people amongst that crowd.
"They'll be identifying mannerisms, looking for what looks out of place. It's the presence of the abnormal, it's the absence of the normal. Amongst that crowd what you'd want to see is a load of smiling faces, people being open and pleasant to one another and eager to see the King.
"Of course, you could have that one person in the crowd who is maybe sweating profusely, may be looking a bit awkward and fidgeting and that's the sort of person where you would get on the radio and say you've got a suspicious individual in the crowd and give a description.
"And then a covert operator or a uniformed officer will go in and have a chat with them."
Aitch said such scenarios, particularly when the person in question is greeting people, are "very difficult" because it "only takes a split second for someone in the crowd to come out with a knife or someone to come out and lunge for whatever reason. And you are on the backfoot immediately."
But he added: "Part of being a policeman, part of the training, is understanding people, understanding mannerisms. So they are accustomed to that role."
As of Friday, more than 30 arrests had been made as part of the operation for a range of offences, Cundy said.
Officers from almost every force in the country are helping police in London, with Met Commissioner Sir Mark Rowley - who only took over the role a week ago - likening the possible total number being deployed to the size of West Midlands or Greater Manchester police.
Armed police, motorbike escort riders, officers carrying out patrols on horseback, dog teams and the marine unit are among the specialist teams involved.
Rooftop snipers would be in place while the cortege is moving, accompanied by a helicopter escort anywhere outside of the capital, Aldworth said.
There are more than 22 miles of barriers in central London alone to control crowds and keep key areas secure.
There is also a considerable police presence in Windsor ahead of the queen's committal later in the day.
Thames Valley Police Assistant Chief Constable Tim De Meyer said more than 2,000 officers will be deployed there in the force's "largest and most significant" operation.
In the Berkshire town, drones have been used to monitor crowds, vehicle barriers are in place and there will be "airport-style" security checks like screening arches and bag searches.
