Police will be putting a ring of steel around Windsor for Prince Philip's funeral this weekend.
Specialist officers and sniffer dogs have been searching phone boxes, post boxes, drains and bins as part of the operation.
Thames Valley Police said it has put a range of visible and covert security measures in place for Saturday, when the duke is to be honoured with a ceremonial royal funeral at St George's Chapel.
These include automated number plate checks, CCTV and barriers to prevent attacks using vehicles, as well as armed and mounted officers on patrol.
And experts say GCHQ could even be listening in to 'chatter' around the world online and on phones, looking out for any spikes in conversations about the funeral.
This comes as Scotland Yard steps up patrols as revellers get set to flock to pub gardens and activists descend on the Capital for another 'Kill the Bill' protest.
Armed Police Officers pictured on patrol outside Windsor Castle today
Thames Valley Police said it has put a range of visible and covert security measures in place for Saturday
Officers will also be patrolling Buckingham Palace and Westminster for the Duke of Edinburgh's funeral.
Police have asked the public not to gather by royal residences, to avoid meeting in large groups, to minimise travel and to make a donation to charity instead of laying floral tributes.
Patrols were also being carried out in Windsor town centre, the Long Walk and Home Park.
Those who did travel to Windsor Castle to lay tributes today were asked to remove any wrapping from flowers and warned they would be removed at the end of the day.
Among the people paying tribute was Professor Chris Imafidon, who met Phillip in 2003 when the Royal visited Queen Mary University in East London.
He said: 'I was nervous about meeting him but he made me feel so relaxed straight away. I told him I was nervous because I was a foreigner and he was a member of the British Royal family.
'But he said to me "I am Greek, I'm a foreigner and I'm a refugee too!"
'I have come to pay my respects because he was such a wonderful person. He was all about education, emancipation, empowerment and equality. He will be missed by many, many people.'
An armed police officer stands guard at the entrance to Windsor Castle
Dance teacher Carole Hellewell, 60, from Farnham, said: 'I was here on Sunday and it was very busy. I've put some flowers down then.
'I've come back with my husband David because our friends in Canada and America asked us to lay some tribute for them too.
'It's a very, very sad time and I think Prince Philip's death will be felt by millions all over the world.'
Publican Robert Gillespie, 62, who runs the Two Brewers Pub next to the castle, has decorated his establishment in red white and blue flowers to mark the Prince's death.
He said: 'I used to see him when he would drive his horse and carriage down here from the Royal Mees towards the Long Walk and he would always give me a wave.
'I'll be closing the pub tomorrow or the day of his funeral also to pay respects to him.'
Only a small number of guests will attend the service tomorrow, with even the Prime Minister stepping aside to make way for an additional family member under the tight rules on numbers.
Children leave a floral tribute to Prince Philip, Duke Of Edinburgh, ahead of his funeral
Those who did travel to Windsor Castle to lay tributes today were asked to remove any wrapping from flowers and warned they would be removed at the end of the day
Funerals of senior royals are years in the planning, with each operation named after a different bridge. The Queen is thought to be London Bridge and the duke is Forth Bridge.
The security operation will focus on preventing risks and 'embarrassment', particularly controlling unauthorised access to the castle.
But police and security services will be alert to any incidents that could prevent a threat - from accidents, public disorder and crime to terrorist attacks.
The potential for knife attacks, shootings and the ability to plant bombs will all be considered.
But officers may be particularly 'alive' to threats drones could pose in disrupting or attacking the ceremony and can draw on technology to try to scupper their flight path or even call in the military.
Richard Aitch, director of operations for firm Mobius International, which offers security services for governments and private companies, said there could be a focus on covert tactics to avoid overshadowing the sombre mood of the day.
He believes that even in a pandemic, the security operation for an event like this would be 'huge', with the cost running 'into the millions'.
It could also involve GCHQ listening in to 'chatter' around the world online and on phones, looking out for any spikes in conversations about the funeral, while vetting would be a 'huge aspect' of the arrangements, he added.
Flowers and condolences from around the Windsor Estate have been moved inside the Castle precinct
The widespread use of face masks amid the pandemic may present challenges to police as they make it harder to identify people, he warned.
Scotland Yard will also be putting more officers on the streets as the country gets set to enjoy the first weekend of relative freedom.
The Metropolitan Police said it anticipates a busy weekend as pubs open for the first weekend since December.
More officers will also be out across London for Prince Philip's funeral.
The force said in a statement: 'Saturday is a significant day for the nation, as His Royal Highness The Duke of Edinburgh is laid to rest in Windsor.
'While this event is not in London, the Met has had a protective security operation in place in the vicinity of Westminster and Buckingham Palace to keep people safe and to deter criminal activity. This highly visible policing operation will continue through the weekend.
'On Saturday afternoon, the Met is aware that a number of people are set to gather in central London as part of a demonstration. Gathering for the purpose of protest can be permitted under the current regulations.
'However, a risk assessment must be completed which sets out how the gathering will be safe and minimise the risk of those taking part transmitting Covid-19.'