US President Donald Trump and First Lady Melania Trump are meeting the Queen on the first day of their state visit to the UK.
The pair are at Buckingham Palace for a private lunch and welcome ceremony.
Mr and Mrs Trump arrived on Air Force One earlier on Monday and were taken to the US ambassador’s home in central London, where they are staying.
Minutes before touching down, Mr Trump criticised Mayor of London Sadiq Khan, with whom he has clashed in the past.
He tweeted that Mr Khan – who had earlier said the UK should “not roll out the red carpet” for Mr Trump – was a “stone cold loser”, but the president added he was looking forward to his visit.
Protests are planned in several UK cities during the three-day visit, including London, Manchester, Belfast, and Birmingham.
Talks between Mr Trump and outgoing Prime Minister Theresa May will begin on Tuesday, with the pair expected to discuss climate change and Chinese technology firm Huawei.
Crowds were gathered outside Buckingham Palace as the president and first lady landed by helicopter shortly after midday.
After a private lunch, the couple will be given a tour of Westminster Abbey and will also join Prince Charles and the Duchess of Cornwall for tea at Clarence House.
Later, the Queen will host a state banquet at Buckingham Palace, also attended by the Duke and Duchess of Cambridge.
The Duchess of Sussex will not attend following the birth of her son Archie, who is less than a month old. On Sunday, Mr Trump denied calling the duchess “nasty”, despite him using the word on tape.
As he stepped onto UK soil, Mr Trump was greeted by US Ambassador to the UK Woody Johnson and Foreign Secretary Jeremy Hunt.
Tory leadership candidate Mr Hunt, who has spoken about the importance of the UK’s relationship with the US, said Mr Trump mentioned to him “some of his very strong views about the Mayor of London” which he had also tweeted.
Mr Trump’s tweet accused Mr Khan of doing a “terrible job” as mayor, adding: “[He] has been foolishly “nasty” to the visiting president of the United States, by far the most important ally of the United Kingdom. He is a stone cold loser who should focus on crime in London, not me.”
At Monday evening’s Buckingham Palace banquet, both the president and the Queen are expected to make a speech to guests, who will include prominent Americans living in Britain.
Labour leader Jeremy Corbyn, House of Commons Speaker John Bercow, and Liberal Democrat leader Sir Vince Cable are all boycotting the state banquet.
Mr Trump’s daughter, Ivanka, arrived in the UK before the president. On Sunday, she posted a picture from outside the Victoria and Albert Museum in west London where she had visited the Christian Dior exhibition.
On Tuesday morning, President Trump and Mrs May will host a business breakfast at St James’s Palace.
Business leaders expected to attend include Barclays CEO Jes Staley, GlaxoSmithKline chief executive Emma Walmsley, BAE Systems chairman Sir Roger Carr and the National Grid’s John Pettigrew.
President Trump will then visit Downing Street for further talks with Mrs May, followed by a joint press conference.
Protesters are organising a “national demonstration” which will start at Trafalgar Square at 11:00 on Tuesday.
Both the Stop Trump Coalition and Stand Up to Trump protest groups said they would be present.
The Met Police said it had “a very experienced command team” leading the operation to deal with the visit.
The police operation for the president’s visit last year was estimated to have cost nearly £18m.
Although the Queen has met 12 of the 13 US presidents who have been in office during her reign, Mr Trump’s state visit to the UK is only the third by a US leader.
George W Bush and Barack Obama are the only other US presidents to be given a state visit.
State visits differ from official visits and are normally at the invitation of the Queen, who acts on advice from the government. The Queen usually receives one or two heads of state per year and has hosted 112 of these visits since becoming monarch in 1952.