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."
A spokesman for Mr Khan said "childish insults" should be beneath the US president, adding: "Sadiq is representing the progressive values of London and our country, warning that Donald Trump is the most egregious example of a growing far-right threat around the globe."
Labour leader Jeremy Corbyn defended Mr Khan, tweeting: "Tomorrow's protest against Donald Trump's state visit is an opportunity to stand in solidarity with those he's attacked in America, around the world and in our own country - including, just this morning, Sadiq Khan."