US President Donald Trump and First Lady Melania Trump have met the Queen on the first day of their state visit to the UK.
The pair went to Buckingham Palace for a private lunch and welcome ceremony.
Mr and Mrs Trump laid a wreath at the tomb of the unknown warrior in Westminster Abbey and are due to attend a state banquet this evening.
Minutes before arriving in the UK, Mr Trump criticised the Mayor of London, with whom he has clashed in the past.
He tweeted that Sadiq 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.
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, before travelling to Buckingham Palace.
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.
The president's visit coincides with the commemorations for the 75th anniversary of the D-Day landings, which the Queen, Mr Trump and other heads of state will attend at Portsmouth on Wednesday.
Crowds were gathered outside Buckingham Palace as the president and first lady landed by helicopter shortly after midday.
The Queen presented Mr Trump with a first edition of Sir Winston Churchill's book The Second World War, from 1959, with gilt decorations and hand-sewn bindings in the colours of the US flag. He was also given a three-piece Duofold pen set decorated with an EIIR emblem, in a design made exclusively for the monarch.
Mrs Trump received a specially commissioned silver box with a handcrafted enamel lid, decorated in royal blue with roses, thistles and shamrocks to represent the ceiling of Buckingham Palace's music room.
After the private lunch, the Queen showed the couple American artefacts and other items from the Royal Collection. In a nod to the US leader's Scottish heritage, he was shown a bolt of Harris tweed.
Mr and Mrs Trump were taken on a tour of Westminster Abbey, where they laid a wreath at the grave of the unknown warrior. Their next stop is Clarence House, where they will join Prince Charles and the Duchess of Cornwall for tea.
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.-BBC
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