US President Donald Trump exited his helicopter at Windsor. He and his wife Melania were greeted by King Charles' son William, the Prince of Wales, and his wife Catherine.
They escorted Trump and Melania over to meet King Charles and Queen Camilla.
Trump, an overt royal fan, has made little secret of his delight at being not just the first US leader but the first elected politician to be invited by a British monarch for two visits.
"I Love King Charles," he posed on his Truth Social account in February. He added on arrival: "I have a lot of things here that ... they warm my heart. It's a very special place."
King Charles and the royal family will roll out the red carpet for the president when he arrives at Windsor Castle, the oldest and largest inhabited castle in the world and family home to British monarchs for almost 1,000 years, with a carriage procession, gun salutes, a military flypast and lavish banquet.
Britain says it will be the largest military ceremonial welcome for a state visit in living memory.
Trump was greeted by Prince William
Trump exited his helicopter at Windsor. He and his wife Melania were greeted by King Charles' son William, the Prince of Wales, and his wife Catherine.
They escorted Trump and Melania over to meet King Charles and Queen Camilla.
Though the bulk of Trump’s visit will be held out of public view, the UK police are taking no chances.
Large crowds are expected at Windsor, where King Charles will host Trump at the world’s largest and oldest inhabited castle in the world, and anti-Trump protesters have said they want to make their views known.
"I'm very content that we have planned a very comprehensive policing and security operation that has taken into consideration just about every eventuality of what could happen," Assistant Chief Constable Christian Bunt of Thames Valley Police told reporters on Monday.
Bunt said there would be a significant, highly-visible police presence.
Airspace over Windsor and Prime Minister Keir Starmer's Chequers country residence will be closed and specialist officers would be ready to deal with any protest that might take place, he added.
What to know about Windsor Castle
Founded by William the Conqueror in the 11th century, it is the oldest and largest occupied castle in the world, according to the Royal Collection Trust. It has been the home of 40 monarchs, including Queen Elizabeth II, who is buried alongside Prince Philip in St. George's Chapel on its grounds.
Nearly one and a half million people visit the castle each year. One highlight for visitors is the Changing of the Guards ceremony, where guards wear iconic red uniforms with black bearskin caps. State banquets are held in St. George's Hall where 160 people can be seated at a 50m long table.
What to know about the carriage procession
Trump and Melania will join King Charles and Queen Camilla, as well as the Prince and Princess of Wales in a carriage procession through Windsor estate, towards the castle.
As the carriage procession moves off, the U.S. and British national anthems will be played.
King Charles and Trump will be in the front coach, followed by Queen Camilla and Melania in the second coach.
US Secretary of State Marco Rubio and Treasury Secretary Scott Bessent will also take part in the procession.
The processional route will be lined by Armed Forces personnel. It will last about 10 minutes.
Welcomed at Winfield House
The Trumps are staying at Winfield House, the US ambassador's residence in central London, as they did during their first state visit.
Located in Regent's Park, US presidents have stayed and conducted diplomatic meetings in the elegant home stretching back to President Dwight Eisenhower.
More recently, President Barack Obama had meetings with Chinese President Hu Jintao and Russian President Dmitry Medvedev there in 2009. Obama also hosted Queen Elizabeth for a dinner.
Winfield House was built by an American, heiress Barbara Hutton, who named it after her grandfather Frank Winfield Woolworth, founder of the famous general stores.
During World War Two, the house was used by an Royal Air Force balloon unit, according to the London Gardens Trust website on the residence.
After the war, the house was in bad shape and Hutton sold it to the US government for the token sum of one dollar, the website adds.
UK hopes visit will cement special relationship
Prime Minister Keir Starmer is hoping to use that sentiment to Britain's advantage as his government seeks to use the trip to cement the two nations' "special relationship", deepen economic ties, secure billions of dollars of investment, discuss tariffs, and press the U.S. president on Ukraine.
While security for the visit will be tight, with a massive police operation in place in Windsor and 1,600 officers deployed in London to deal with a protest by the "Stop the Trump Coalition", Trump's arrival prompted a protest in Windsor.
Police said four people were arrested on Tuesday following a projection of images of Trump alongside late sex offender Epstein onto one of Windsor Castle's towers. Trump was not in Windsor at the time.
The arrests were for malicious communications following an "unauthorised projection", which police described as a stunt.
Starmer last week sacked Peter Mandelson as UK ambassador in Washington over his ties to Epstein, and it could lead to awkward questions for Starmer and Trump, whose own relationship with the financier has also come under scrutiny.
Tech pact to boost AI, Quantum computing
The visit has already delivered a new technology pact between the two countries which will boost ties in AI, quantum computing and civil nuclear energy. Microsoft has pledged 31 billion pounds ($42 billion) in UK investments.
Starmer will also be looking for further progress on tariffs.
"Basically, I'm there also on trade. They want to see if they can refine the trade deal a little bit. We made a deal, and it's a great deal. And I'm into helping them," Trump said when he left the White House for Britain on Tuesday.
"They'd like to see if they could get a little bit better deal, so we'll talk to them."