US President Donald Trump arrived in Britain on Tuesday for an unprecedented second state visit, with the UK government rolling out a royal red carpet welcome to win over the mercurial leader.

"A lot of things here warm my heart," the 79-year-old Republican told reporters after he arrived with First Lady Melania Trump. "It's a very special place."

King Charles III will host Trump at Windsor Castle for a lavish banquet and carriage ride on Wednesday, before Trump meets Prime Minister Keir Starmer at his country residence on Thursday.

In a sign of the pomp and pageantry to come, a guard of honour greeted the Trumps as they stepped off Air Force One at Stansted Airport near London.

Trump then expressed his admiration for the British monarch.

"He's been a friend of mine for a long time, and everyone respects him, and they love him," Trump said as he arrived by helicopter at Winfield House, the US ambassador's residence in London, where he will spend the night.

"Tomorrow's going to be a very big day."

Trump is the first US president to be invited for two state visits, after previously being hosted by Queen Elizabeth II during his last term in office in 2019 – as Britain tries to keep the so-called special relationship alive.

The British government has been unapologetic about its efforts to get on the right side of Trump, whose mother was Scottish and who owns a number of golf courses in the UK.

But the British public will be kept far away from Trump, with the visit taking place entirely behind closed doors and heavy security.

A large anti-Trump demonstration has been called in London on Wednesday, while protesters projected images of Trump and convicted sex offender Jeffrey Epstein onto the towers of Windsor Castle late on Tuesday.

The shadow of the Epstein scandal has hung over Trump for weeks after his administration declined to release files on the issue.

It has also embroiled British Prime Minister Keir Starmer, who sacked his ambassador to Washington Peter Mandelson on the eve of the visit over the envoy's emails to Epstein.

Starmer's government is however counting on the sheer scale of the state visit to keep Trump onside, despite his unpredictability over everything from tariffs to Ukraine and Gaza.

In Windsor on Wednesday, Trump will get a ceremonial carriage ride and a flypast by both UK and US jets in what officials call the largest military show for a state visit in living memory.

In the evening King Charles will host Trump for a lavish state dinner where both men will give speeches. (AFP)