The first coronation in 70 years will take place later, with King Charles III and the Queen Consort preparing for their historic carriage ride to Westminster Abbey.
Large crowds with Union jacks have gathered along the procession route, with many camping out overnight.
Thousands of military personnel are arriving in London to take part, and a large security operation is in place.
The Coronation service will be seen first-hand by 2,300 guests.
Some 100 heads of state will attend the service, which will begin around 11:00 BST and last for almost two hours.
A key theme of the Coronation for King Charles will be service and in his first prayer when he reaches the Abbey he will say: “I come not to be served, but to serve.”
Following the ceremony, the King and Camilla will leave Westminster Abbey to return to Buckingham Palace.
The guest list also includes Prince Harry, the Duke of Sussex, who arrived from the United States on Friday on a commercial flight.
It will be the first time since the release of the duke’s memoir that he will have been seen in public with his brother, Prince William, the Prince of Wales.
It is thought Prince Harry may fly back to the US a matter of hours after the ceremony to rejoin his wife Meghan, with their son Archie celebrating his birthday the same day.
More than 5,000 armed forces personnel are arriving by train into London Waterloo station from garrison towns across the country, before marching over Westminster Bridge.
It is the largest movement of military personnel on Britain’s railways since Winston Churchill’s funeral in 1965, according to Network Rail.
Thousands of people have gathered along the procession route despite the forecast of rain later.
Among them is 10-year-old Hudson, who in the last few years has witnessed Prince Harry and Meghan’s wedding, the Jubilee and the late Queen’s lying-in-state.
“I’m extremely excited for today,” he told the BBC. “I’m excited to see the King in the gold carriage, and hopefully the aeroplanes.”
Charles became king of the United Kingdom and 14 other realms in September, when his mother Elizabeth died after 70 years on the throne. Months of intense planning have gone into the coronation celebrations – the 40th to take place at Westminster Abbey since 1066.
The Archbishop of Canterbury, Justin Welby, will preside over the service, with guests ranging from US First Lady Jill Biden to President Macron of France and entertainers Ant and Dec.
He will be assisted by the Archbishop of York, Stephen Cottrell, who has already been seen arriving at Westminster Abbey.
Olena Zelenska, wife of the Ukrainian president Volodymyr Zelensky, met Catherine, Princess of Wales at a pre-Coronation reception at Buckingham Palace on Friday.
Charles was photographed sharing warm words with the First Lady of Ukraine, and greeting Mary, Crown Princess of Denmark, with a handshake and a kiss on the cheek.
There has been controversy over whether people at home were being asked to pledge their allegiance to the King.
The Church of England has made clear this is entirely optional and people might instead have a “private moment of reflection”.
The high point of the ceremony will come when the St Edward’s Crown is placed on the King’s head, a moment that will be marked by the Abbey bells being rung and a gun salute in nearby Horse Guards Parade.
Camilla will be crowned alongside Charles – and after the couple’s long and often complicated relationship, she will now be officially described as “Queen Camilla”.
The ceremony will emphasise diversity and inclusion, with more multi-faith elements than any previous coronation, with contributions from Jewish, Muslim, Buddhist and Sikh representatives.
A Bible lesson will be read by Prime Minister Rishi Sunak, who is Hindu, and music will be sung in Welsh and Scottish and Irish Gaelic.
There will be women bishops taking part in the service for the first time in a coronation service that goes back almost a thousand years.
After the service, at around 13:00 BST, King Charles and Queen Camilla will travel in the Gold State Coach back to Buckingham Palace, in a spectacular mile-long (1.6km) procession, with 4,000 soldiers and 19 military bands.
Meticulous rehearsals for the procession were carried out by marching around a replica route with landmarks such as the Cenotaph marked out with traffic cones.
When they reach the Palace, it remains uncertain who will be seen with the King and Queen for the traditional balcony appearance.
There are plans for a flypast when the senior royals are on the Palace balcony, but there will be concerns about the weather, with a forecast for cloud and showers.
The Coronation has also drawn a small group of protesters from Republic, the group campaigning for the abolition of the monarchy, who are assembling in Trafalgar Square.
There will be a massive security operation, with the Metropolitan Police putting 11,500 officers on duty in what it says will be its biggest ever single-day deployment.
Anti-monarchy groups have defended their right to protest, but the police have warned that “tolerance for any disruption, whether through protest or otherwise, will be low”.
The guest list has also been disputed, with criticism of the presence of Chinese vice-president Han Zheng, accused of presiding over a crackdown on civil liberties in Hong Kong.
But this will be a ceremony played out before a huge global audience, with TV crews from all over the world arriving in London.
They will see pageantry, religious symbolism and ancient traditions, with King Charles III crowned in a 700-year-old Coronation chair, in a ceremony that most people will never have seen before in their lives.
