William, who met the couple on behalf of his father King Charles and who last visited Japan in 2015, travelled with Naruhito and Masako to Horse Guards Parade for the official ceremonial welcome.
The state visit has been a long time coming. It was originally billed for 2020 but was postponed due to the COVID-19 pandemic.
HELLO!'s Royal Editor, Emily Nash, noted: "The royal family are rolling out the red carpet for the Emperor and Empress and their delegation and it's a really important display of soft power. Japan is a key strategic partner for the UK so this kind of welcome and hospitality will really help to boost that relationship.
"Sending the Prince of Wales to escort the Emperor and Empress to the parade ground further shows how important their visit is considered by the UK government."
Emperor Naruhito and Empress Masako arrived in the UK on Saturday
The UK's upcoming General Election on 4 July has also meant that some aspects of the state visit have been cut while party leaders are busy campaigning, such as the usual Downing Street talks with the Prime Minister, a speech to the Palace of Westminster by the visiting head of state, and meetings with opposition leaders.
A Palace spokesman said this week's programme had been "slightly adapted", adding: "As a general principle, it has of course been adapted as a result of the current pre-election period of sensitivity."
Princess Anne, 73, would normally participate in the state visit too, however following Sunday evening's incident at her home, Gatcombe Park, which involved a horse and left her with minor injuries to her head and concussion, Anne is no longer able to travel to London.
She is being monitored at Southmead Hospital in Bristol where she is expected to make a swift and full recovery.
Royal latest
Take a look at the best photos from day one of the Japanese state visit below…
King Charles and Queen Camilla were the first to arrive at Horse Guards Parade, ready to welcome their Japanese guests. The royals greeted a line of waiting dignitaries, who included Prime Minister Rishi Sunak and military top brass.
Camilla, 76, looked typically elegant in a white crepe silk summer dress with black piping by Anna Valentine. She paired it with a black and white hat by Philip Treacy, and accessorized with a diamond flower brooch.
Prince William met Emperor Naruhito, 64, and ruler of the Chrysanthemum throne, and his wife Empress Masako, 60, at their hotel before accompanying them to Horse Guards Parade on Tuesday morning.
The red carpet was rolled out for the Japanese royals and the formal state occasion included all the traditional pomp and pageantry expected of a ceremonial welcome.
King Charles and Queen Camilla greeted their guests at The Royal Pavilion.
The state visit was arranged at the request of the British government.
A Japanese Foreign Ministry official said the emperor and empress's visit would not be a political one and it was hoped it would forge "friendly relations across generations" between Japan's imperial family and the British royal family.
The incoming state visit marked the King's first since his cancer diagnosis was made public earlier this year.
The emperor, accompanied by King Charles, then inspected the Guard of Honour, formed of the 1st Battalion Welsh Guards with the Band of the Welsh Guards.
It was then time for the royals and their visitors to ride in a carriage procession along The Mall to Buckingham Palace, where they were met by a second Guard of Honour formed of the 1st Battalion Welsh Guards.
Part of the engagement included laying a wreath at the Grave of the Unknown Warrior.
Japan state banquet
The final engagement of day one will see the royals dress to the nines for a lavish state banquet at Buckingham Palace.
Both the King and the emperor will give speeches at the start of the dinner, which will see other notable guests in attendance, including Prime Minister Rishi Sunak and Labour leader Sir Keir Starmer, who are expected to take a break from the campaign trail to attend.
Prince William, who is president of the Football Association, is also expected to be at the state banquet, although it clashes with England's last group game in the Euro 2024 tournament.
Other engagements during the state visit
The state visit will cover three days. On Wednesday, Emperor Naruhito will visit The Francis Crick Institute, the UK's flagship biomedical research centre.
The Institute supports an innovative UK-Japan research partnership which covers a range of public health issues, including cancer, vaccines, and the role of genetics in infectious diseases.
That evening, the Emperor of Japan will be joined by the Duke and Duchess of Edinburgh at a banquet at the Guildhall, given by the Lord Mayor and City of London Corporation. The Lord Mayor and the emperor will both make speeches at the end of the banquet.
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Thursday, the final day of the state visit, will see King Charles and Queen Camilla formally bid farewell to their visitors in the morning.
The emperor and empress will then travel to Young V&A, where the museum's Japan: Myths to Manga exhibition is currently on display.
Emperor Naruhito will also privately travel to St. George's Chapel, Windsor Castle, to lay a wreath on the tomb of the late Queen Elizabeth II.
He will then pay a visit to the Royal Botanic Gardens in Kew to tour the historic Temperate House, which is currently displaying bronze bonsai sculptures by artist Marc Quinn, as well as Kew's regular collection of bonsai trees.
On the final day of their visit to the UK on Friday, Naruhito and Masako will travel to Oxford for a private programme of engagements, including a visit to the colleges where Their Majesties studied.
At the end of their visit, The Lord Chamberlain will bid farewell to the couple on behalf of the King, before they depart from RAF Brize Norton.
Naruhito and Masako arrived in the UK last Saturday, three days prior to the state visit. They undertook a private schedule of engagements, including visits to Japan House and the Thames Barrier.
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