Donald Trump was treated to a sumptuous royal state banquet in Windsor Castle on Wednesday, rounding off the US president's first full day of a historic second state visit to Britain.

Before some 160 guests at the start of the lavish event, designed to play into the mercurial American leader's love of pomp and pageantry, King Charles praised what he called Trump's "personal commitment" to seeking peace.

For his part Trump described the visit as "one of the highest honours of my life," paying tribute to the transatlantic relationship by calling the UK and the United States "two notes in one chord... each beautiful on its own, but really meant to be played together."

Trump and his wife Melania's welcome earlier featured gun salutes, soldiers on horseback and a flypast by the Red Arrows as the visit unfolded far from the British public and marching protesters.

The King and Trump laughed and joked as the US leader inspected troops at Windsor, west of London, in an elaborate spectacle featuring a carriage procession.

Some 120 horses and 1,300 members of the British military – some in red tunics and gold plumed helmets – feted Trump during a ceremonial guard of honour that UK officials called the largest for a state visit to Britain in living memory.

The president and first lady were greeted on arrival by heir-to-the-throne Prince William and his wife, Catherine at Windsor after stepping off the Marine One helicopter under grey skies.

Inside a ring of steel and out of sight from thousands of demonstrators shouting anti-Trump slogans in London, William and Catherine then walked Trump and his wife a short distance to meet the waiting Charles and Queen Camilla.

A 41-gun salute was fired simultaneously from six First World War-era guns on the castle's east lawn, as a similar display took place at the Tower of London, in the capital.

The Trumps and the royals were then taken in a black and gold coach, surrounded by cavalry, through the Windsor estate towards the nearly 1,000-year-old castle, where Trump and Charles inspected the guard of honour.

The two also exchanged gifts, with the royals giving Trump a book to celebrate the 250th anniversary of the US declaration of independence next year, and Trump gifting Charles a replica of a sword owned by President Dwight Eisenhower.

Trump is the first US president to receive two state visits, after his previous one with Queen Elizabeth II in 2019.

At Windsor, he laid a wreath on the tomb of Elizabeth, Charles's mother, who died in 2022, and viewed a collection of US-related items, including a letter sent from President Abraham Lincoln to Queen Victoria in 1862 after the death of Prince Albert.

The Trumps also had a private meeting with William and Catherine, described by their Kensington Palace office as "warm and friendly." (AFP)