US President Donald Trump called on Queen Elizabeth II at Buckingham Palace on Monday as he landed here on a three-day state visit to strengthen the bilateral ties overshadowed by the Brexit crisis and his hostility with London's Pakistani-origin mayor whom he termed "nasty" and a "stone cold loser".
The 93-year-old monarch welcomed the US President and the First Lady at Buckingham Palace with a ceremonial guard of honour welcome ceremony, which was followed by a private lunch at the palace in central London.
Trump and Melania were received by Prince Charles after their helicopter Marine One landed on the West Lawn of the palace on Monday afternoon.
As the Trumps landed, they were greeted with two 41-gun salutes fired simultaneously in The Green Park by the King's Troop Royal Horse Artillery and at Tower of London by the Honourable Artillery Company.
Apart from meeting members of the Royal Family, President Trump is expected to discuss climate change and Chinese technology firm Huawei during his talks with outgoing Prime Minister Theresa May on Tuesday.
Earlier, just moments before his Air Force One touched down at Stansted Airport, the US President took to Twitter in his characteristic style to attack London's Mayor Sadiq Khan with whom he has clashed in the past, terming him "nasty" and a "stone cold loser".
"Sadiq Khan who by all accounts has done a terrible job as Mayor of London, 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," he said.
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Trump compared the 48-year-old Khan with New York Mayor de Blasio as having done an equally "terrible job".
Khan's spokesperson responded by saying that "childish insults" should be "beneath the President of the United States".
"Sadiq is representing the progressive values of London and our country," the spokesperson said while adding that "Donald Trump is the most egregious example of a growing far-right threat around the globe".
UK foreign secretary Jeremy Hunt, who was at Stansted Airport to receive the US leader, also said that one of the first things Trump spoke about was some of his "strong views" about the London Mayor.
Over the weekend, Khan had branded Trump a fascist and a divisive figure who should not be accorded the pomp and ceremony associated with a UK state visit.
Protests are planned across the UK during the visit, including in London, Manchester, Belfast, and Birmingham and a giant balloon depicting Trump as a baby is to take to the skies once again over London, in a repeat of his last UK visit in 2018.
On Monday, the US President will be given a tour of Westminster Abbey and will also meet Prince Charles and the Duchess of Cornwall for tea at their Clarence House residence in London.
In the evening, the Queen will host the ceremonial state banquet at Buckingham Palace for Trump and Melania. She will be joined by the Duke and Duchess of Cambridge, William and Kate.
The agenda for Trump's weeklong tour of Europe includes the World War II D-Day commemoration ceremonies on both sides of the English Channel and his first presidential visit to Ireland, which will include a stay at his coastal golf club.
British Prime Minister May, who is to preside over bilateral talks at Downing Street, will formally resign as Prime Minister a day after the US President leaves London on Thursday.
A planned lengthy bilateral meeting with May has been taken off the agenda, with the outgoing British Prime Minister set to meet the US President only briefly.
Opposition Labour Party leader Jeremy Corbyn, House of Commons Speaker John Bercow, and Liberal Democrat leader Vince Cable are all boycotting the state banquet.
Meghan Markle, the Duchess of Sussex, will also not attend following the birth of her and Prince Harry's son Archie, who is less than a month old.
The American-born former actress has previously made critical remarks about Trump and calling him a divisive figure.
President Trump will be staying at the US ambassador's residence, Winfield House, in central London's Regent's Park as opposed to a customary state visit stay at Buckingham Palace which is currently undergoing essential repairs.
George W Bush and Barack Obama are the only other US Presidents to be accorded a state visit welcome to the UK. State visits differ from official visits and are normally at the invitation of the Queen, who acts on advice from the government.