LONDON - President Donald Trump met Queen Elizabeth II and Prince Charles at Buckingham Palace on Monday, beginning three days of royal feting and carefully calibrated diplomacy. The royals had lunch and a tea with Trump. They showed him old paintings of George Washington and an honor guard in scarlet tunics.

But first, Trump mocked the relatively popular London mayor as a "stone cold loser" - and short in stature. And then Trump complained at length on Twitter about CNN news coverage of his trip, which had only just begun.

The president and first lady Melania Trump were greeted at the satellite Stansted Airport by Foreign Secretary Jeremy Hunt - but not by Prime Minister Theresa May, who is being ousted over Brexit. Trump's official state visit coincides with a furious contest to replace May. It is all a bit awkward. Not that the British will mention it, much.

At Buckingham Palace, the president appeared to enjoy himself immensely as he strode the ranks of the queen's guards decked out in black bearskin hats and scarlet jackets.

These were the two 41-gun salutes.

Trump didn't receive that traditional royal welcome at Horse Guards Parade or a gold carriage procession down the Mall - security concerns were cited. He also won't get to stay overnight at the Buckingham Palace - the palace cited renovations. He'll be staying at Winfield House, the U.S. ambassador's residence, instead.

After lunch, the Queen escorted the president and first lady into the palace picture gallery to see items of "historical interest" to the United States. A portrait of George Washington was visible on the far table, according to reporters at the edge of the scene.

Also on display was a copy of the Declaration of Independence - under the heading - "A Tale of Two Georges: King George III and George Washington."

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The photographers caught a fleeting glimpse of Prince Harry with Trump's daughter Ivanka at the back of the gallery. Absent was Harry's wife, Meghan, who just had a baby, and whom Trump last week called "nasty" for criticizing him during the 2016 campaign, when she was pre-royal and still actress Meghan Markle.

"The Queen and the entire Royal family have been fantastic," Trump tweeted after his afternoon with them. "Tremendous crowds of well wishers and people that love our Country. Haven't seen any protests yet, but I'm sure the Fake News will be working hard to find them."

Britain is deeply anti-Trump. An Ipsos MORI poll last year found 19% of Brits had a favorable view of him, while 68% were unfavorable.

Some of that sentiment was visible on Monday. Amnesty International unfurled banners from London bridges with the captions "Resist Trump" "Resist Racism" and "Resist Sexism."

But the major demonstration is scheduled for Tuesday, when of thousands of protesters are expected to pack London's Trafalgar Square. A giant balloon depicting Trump as a diaper-clad baby is expected to take flight, and opposition Labour Party leader Jeremy Corbyn is scheduled to address the crowds.

London Mayor Sadiq Khan, a Muslim and the son of a Pakistani immigrant bus driver, has become the rhetorical leader of London's resistance to the president. He vocally objected to "rolling out the red carpet" for Trump with a state visit. And, writing in the Guardian newspaper Sunday, he said Trump used the language of the "fascists of the 20th century."

As his plane was touching down, Trump responded by criticizing Khan's record as mayor and mocking his height. (Khan is 5-foot-6.) Trump also misspelled his name.

"@SadiqKhan, 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," Trump wrote on Twitter.

"Kahn [sic] reminds me very much of our very dumb and incompetent Mayor of NYC, de Blasio, who has also done a terrible job - only half his height. In any event, I look forward to being a great friend to the United Kingdom, and am looking very much forward to my visit. Landing now!"

Khan's spokesman responded to Trump's Twitter attack Monday, saying that "childish insults" should be "beneath the president of the United States."

Khan posted a video message on Monday afternoon, which began: "President Trump, if you are watching this, your values, and what you stand for, are the opposite of London's values and the values of this country."

Khan posted a video message on Monday afternoon, which began: "President Trump, if you are watching this, your values, and what you stand for, are the opposite of London's values and the values of this country."

Khan also criticized Trump for inserting himself into the internal politics of Britain.

Trump arrived at a sensitive political moment in Britain. May will officially leave her post as leader of the Conservative Party on Friday. The contest to replace her is in full swing, with at least 13 candidates known to have thrown their hat in the ring. The winner is expected to be announced in July.

As Trump was landing, Boris Johnson, Britain's former foreign secretary, officially launched his leadership bid with a campaign video that showed him out meeting voters. Trump told the Sun newspaper on Friday that Johnson "would be excellent" as prime minister. He also spoke well of Jeremy Hunt, the current foreign secretary who is also gunning for the top job. "Yep, like him," Trump said.

Before meeting the queen, Trump was at the London residence of U.S. Ambassador Woody Johnson, where he was possibly watching a little telly.

Trump tweeted, "Just arrived in the United Kingdom. The only problem is that @CNN is the primary source of news available from the U.S. After watching it for a short while, I turned it off. All negative & so much Fake News, very bad for U.S. Big ratings drop."

If he had worked the remote, he could have presumably tuned into some of the more watched television stations in the country, which include the BBC, Channel 4, ITV, and Sky.

A banquet with the queen was scheduled for Monday evening.

These type of encounters with the British royal family, whether state visits or working visits, are minutely choreographed affairs. But there are usually unpredictable moments - and many wonder if there could be more so with this president than most.

During Barack and Michelle Obama's visit in 2009, the queen and the first lady struck up a conversation about standing around in uncomfortable shoes. Michelle then laid a hand on the queen's shoulder, which is not done. The queen responded by putting her arm on Michelle's back. It became one of the defining images from that trip.

Then there was Trump's working visit to Britain in July 2018 when he appeared to walk in front of the queen during an inspection of the royal guard, causing a social media uproar.

Woody Johnson, who in addition to being U.S. ambassador owns the New York Jets football team, called the state visit "very significant."

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"He knows the security and prosperity of the U.S. is directly linked to the security and prosperity of the U.K. The special relationship will be a huge focus as we remember D-Day," Johnson told the BBC.

"When I last spoke to him he was extremely enthusiastic. The president's mother was born here, and this is part of his DNA. Everything he is about revolves around this relationship. It could not be more important," Johnson said.

Johnson said the Trump administration was looking forward to signing a U.S.-British trade deal - though in the past the ambassador warned that the exit deal with the E.U. that May tried to pass through Parliament could threaten an agreement with Washington.

More controversial, Woody Johnson said Sunday that a future trade deal with the United States would include British health care, specifically the social medicine program called the National Health Service. Although Britons often complain about it, the program has broad support.

Many Britons have expressed fear that the United States has designs on profiting from the NHS.

Asked if British consumers would buy U.S. meat and vegetables, which have less strict regulations over chemicals, Johnson said British consumers would make their own choices.

It's unclear how much Trump will be able to focus on the British pageantry rather than the political drama back in the United States, where the president faces a burgeoning trade war with Mexico, intensifying congressional investigations and growing calls for impeachment.

Trump's unprompted attack on New York Mayor Bill de Blasio - who is one of two dozen Democratic candidates seeking to unseat Trump - offers a signal that the president will continue to engage in domestic politics even while on foreign soil.

While in Japan last month, Trump used the words of North Korean dictator Kim Jong Un to attack another rival, former vice president Joe Biden - calling him a "low IQ individual."

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The Washington Post's Karla Adam contributed to this report.

This article was written by William Booth, Toluse Olorunnipa and Anne Gearan, a reporter for The Washington Post.