At Buckingham Palace, King Charles holds the “reception of the century.”

King Charles III and Queen Consort Camilla have had their first guest arrive which have so far included US President Joe Biden, who arrived in a private car, and President of Ireland Michael D Higgins who was dropped off from a coach with the former King and Queen Consort of Spain, Juan Carlos and Sofia.

The royal couple will soon greet hundreds of dignitaries, from world leaders including US President Joe Biden, New Zealand’s prime minister Jacinda Ardern and Canadian premier Justin Trudeau.

This is along with senior members of the British royal family, ambassadors from almost all nations, and foreign royals including Japan’s Emperor Naruhito and the Kings and Queens from Holland, Norway and Spain.

Many dignitaries have already been viewing the Queen’s coffin in the ancient heart of Parliament after being given a VIP timeslot – raising the prospect that mourners who have queued through the night will pay their respects with leaders of the free world.

The PM Liz Truss and Government ministers including Jacob Reese-Mogg and James Cleverly were also spotted this evening heading towards tonight’s glittering state reception which will take place in the picture gallery and state apartments and include drinks and canapes.

Guests are asked to wear lounge suits and morning dress rather than ball gowns and white tie as would be expected at a usual state event at the King’s London home.

It is thought Prince Harry and his wife Meghan Markle received an invitation to the event, hosted by King Charles and the Queen Consort, earlier in the week.

But the couple are now unlikely to attend after officials at Buckingham Palace insisted the reception was for working royals only, it is understood.

The confusion over Harry and Meghan’s invitation points to issues with communication between the Californian-based couple and the Royal Family, according to the Daily Telegraph. It follows an apparent U-turn over Harry’s right to wear military uniform despite being a non-working royal.

The Palace is understood to have intervened to allow Harry to wear his regalia to a 15-minute vigil at Westminster Hall yesterday. Harry had previously said he would wear a morning suit to all the funeral events.

The funeral of the only monarch most Britons have known involves the biggest security operation London has ever seen.

Mayor Sadiq Khan says tomorrow’s state funeral is an ‘unprecedented’ security challenge, with hundreds of thousands of people packing central London and a funeral guest list of 500 emperors, kings, queens, presidents, prime ministers and other leaders from around the world.

‘It’s been decades since this many world leaders were in one place,’ Mr Khan said. ‘This is unprecedented … in relation to the various things that we’re juggling.’

‘There could be bad people wanting to cause damage to individuals or to some of our world leaders,’ he told The Associated Press. ‘So we are working incredibly hard – the police, the security services and many, many others – to make sure this state funeral is as successful as it can be.’

Metropolitan Police Deputy Assistant Commissioner Stuart Cundy said the ‘hugely complex’ policing operation is the biggest in the London force’s history, surpassing the London 2012 Olympics.

‘Our response here in London will be proportionate, it will be balanced, and officers will only be taking action where it is absolutely necessary,’ he said.

Metropolitan Police Commissioner Mark Rowley said the goal was to keep the event safe, ‘and try to do it in as unobtrusive a way as possible, because this is obviously a solemn occasion.’

More than 10,000 police officers will be on duty Monday, with London officers supplemented by reinforcements from all of Britain’s 43 police forces. Hundreds of volunteer marshals and members of the armed forces will also act as stewards along the processional route.

They are just the most visible part of a security operation that is being run from a high-tech control center near Lambeth Bridge, not far from Parliament.

Street drains and garbage bins are being searched and sealed. Tomorrow there will be police spotters on rooftops, sniffer dogs on the streets, marine officers on the River Thames and mounted police on horseback.

Flying drones over Central London has been temporarily banned, and Heathrow Airport is grounding scores of flights so that aircraft noise does not disturb the funeral service.

Authorities face the challenge of keeping 500 world leaders safe, without ruffling too many diplomatic feathers. Presidents, prime ministers and royalty will gather offsite before being taken by bus to the abbey – though an exception is being made for Mr Biden, who is expected to arrive in his armored limousine, known as The Beast.

Another challenge is the sheer size of the crowds expected to gather around Westminster Abbey and along the route the coffin will travel after the funeral, past Buckingham Palace to Hyde Park. From there it will be taken by hearse about 20 miles to Windsor, where another 2,000 police officers will be on duty.

The Queen is due to be interred in St George’s Chapel at Windsor Castle alongside her husband Prince Philip, who died last year aged 99.