United States President Donald Trump met King Charles III on Wednesday during his unprecedented second state visit to the United Kingdom, enjoying royal pageantry amid protests and heightened security, as major multibillion-dollar UK-US investment plans were unveiled. On Thursday, he is scheduled to hold high-stakes talks with UK Prime Minister Keir Starmer.

After President Trump and First Lady Melania Trump spent Tuesday night at the US ambassador's residence in central London, the king and the royal family gave the president a full red-carpet welcome at Windsor Castle, the world's oldest and largest occupied castle and the monarchs' home for nearly a millennium.

The ceremonial events around midday included a carriage procession, gun salutes and a military flypast. The president and first lady then had lunch with the wider royal family before laying a wreath at Queen Elizabeth II's tomb in St George's Chapel. Later, a lavish evening banquet was held.

The king's special guests were due to stay at the castle overnight, and on Thursday, UK Prime Minister Keir Starmer will host Trump at Chequers, his countryside retreat in Buckinghamshire, northwest of London before the president and his wife end their trip and return to the US.

A self-professed royal enthusiast, Trump has made no secret of his delight at being not only the first US leader but also the first elected politician invited by a British monarch for two visits.

Starmer seeks to harness that sentiment to cement the "special relationship", deepen trade ties, secure billions in investment, discuss tariff s, and press the US on Ukraine, reported Reuters.

To coincide with the visit, Microsoft announced it will invest 22 billion pounds ($30 billion) in the UK's AI sector — its largest commitment outside the US — as part of a 31-billion pound "Tech Prosperity Deal".

Alongside Google, Nvidia and others, the package focuses on UK-based AI infrastructure, chiefly new data centers, and includes Microsoft joining a powerful new supercomputer project in Loughton, Essex, which was first announced in January.

Microsoft CEO Satya Nadella told the BBC he expects AI to lift UK productivity and growth within about five years, while cautioning against hype and noting the technology's heavy energy demands.

He said the benefits, particularly for healthcare, public services and business productivity, justify the push, and compared its economic impact to the personal computer's rise, roughly a decade after it began to scale in the 1990s.

"It may happen faster, so our hope is not 10 years but maybe five," he said.

"Whenever anyone gets excited about AI, I want to see it ultimately in the economic growth and the GDP growth."

Police arrested four people on suspicion of malicious communications after images of the late sex off ender Jeffrey Epstein alongside Trump were projected onto Windsor Castle's walls on Tuesday night.

Police also arrested two men on Tuesday for flying drones near Windsor Castle in restricted airspace. Flight restrictions are in place through to Thursday over Windsor and Chequers.

Thames Valley Police monitored Windsor from the air, deployed armed officers on the streets, and patrolled the River Thames as part of an expanded security operation.

Dozens of protesters gathered near Windsor Castle, chanting "Dump Trump" and "Trump go home" on Wednesday, with a larger demonstration taking place in central London.