WORLD leaders including King Letsie III and top foreign dignitaries have flocked to Buckingham Palace to pay their respects to King Charles ahead of the Queen's funeral on Monday - in what is being described by some as the 'reception of the century'.

During one of the largest gatherings of world leaders seen in recent years the newly crowned Monarch and his wife, Queen Consort Camilla, greeted the likes of US President Joe Biden and wife Jill as well as President Emmanuel Macron and his wife Brigitte, for a glittering but sombre event.

Such a reception involving world leaders and British royals has not seen since the death of Queen Elizabeth II's father George VI in 1952, more than 70 years ago.

More recently, but not on such a grand scale, the now late Queen attended the G7 in Cornwall where she met and took part in pictures with the G7 leaders.

President of Ireland Michael D Higgins was seen being dropped off from a coach along with the former King and Queen Consort of Spain, Juan Carlos and Sofia and Ursula von der Leyen, the president of the European Commission, before the British royal couple met hundreds of dignitaries, including New Zealand's Prime Minister Jacinda Ardern and Canadian Premier Justin Trudeau. 

The Prince and Princess of Wales and other working members of the royal family including the Earl and Countess of Wessex, the Princess Royal and Vice Admiral Sir Tim Laurence, and the Duke and Duchess of Gloucester joined Charles and the Queen Consort in mingling with guests including Japan's Emperor Naruhito and the Kings and Queens from Holland, Norway and Sweden. 

Many dignitaries had already been viewing the Queen's coffin in the ancient heart of Parliament after being given a VIP timeslot as ordinary mourners who have queued through the night pay their respects with leaders of the free world. 

The Prime Minister Liz Truss and Government ministers including Jacob Rees-Mogg and James Cleverly were also spotted heading towards tonight’s glittering state reception which took place in the picture gallery and state apartments and featured drinks and canapes. 

Many have also compared the state funeral of the Queen, which will see 1,000s world leaders attend, to Nelson Mandela's memorial service in 2013 in South Africa which had to take place in a stadium due to the huge amount of world mourners. 

King Abdullah II and Queen Rania of Jordan also arrived in their own vehicle at the Grand Entrance and were followed by their son, Hussein, Crown Prince of Jordan, who also took his own vehicle.

Moments later, the Emir of Qatar, Tamim bin Hamad Al Thani, arrived.

King Hamad bin Isa Al Khalifa and the Crown Prince of Bahrain, Salman bin Hamad Al Khalifa, also joined the guests, and the Crown Prince of Kuwait, Nawaf Al-Ahmad Al-Jaber Al-Sabah, soon followed.

The Sultan of Oman was also seen arriving at the palace, while Ruler of Dubai Mohammed bin Rashid Al Maktoum followed shortly afterwards.

King Felipe and Queen Letizia of Spain, King Carl XVI Gustaf and Queen Silvia of Sweden, and Queen Margrethe II of Denmark were among those arriving on coaches.

European Commission president Ursula von der Leyen and president of the European Council Charles Michel arrived on coaches with several leaders of Commonwealth nations at the Grand Entrance.

They were joined by New Zealand Prime Minister Jacinda Ardern, who walked into the palace followed by Canadian Prime Minister Justin Trudeau, who smiled briefly at journalists.

Brazilian President Jair Bolsonaro was also among guests at the event which took place across the picture gallery, the white drawing room, the blue drawing room and the music room.

The Prince and Princess of Wales and other working members of the royal family including the Earl and Countess of Wessex, the Princess Royal and Vice Admiral Sir Tim Laurence, and the Duke and Duchess of Gloucester joined Charles and the Queen Consort in mingling with guests. 

Guests were 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. But some were allowed to opt for traditional clothing. 

And reports suggest that the Duke and Duchess of Sussex were 'uninvited' to a state reception for world leaders and foreign royals this evening. 

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.