"Yer Majesty, Melania and I are profoundly honored to be your guests for this historic state visit." U.S. President Donald Trump hailed Queen Elizabeth II as a symbol of British strength on Monday "A great, great woman." ...during a toast at a banquet in Buckingham Palace.
Outside, large crowds of protesters booed the American president.
Trump reignited a feud with London's mayor, Sadiq Khan, even before setting foot on British soil, calling him a "stone cold loser" in a tweet after the mayor criticized the British government's decision to offer a state visit to the sitting president.
In the tweet, Trump also said the mayor has been foolishly 'nasty', calling to mind a comment he made ahead of the trip about Dutchess Meghan Markle - a comment he denies having made.
The president and First Lady Melania began the three-day visit with a ceremonial welcome.
Meanwhile, activists have decorated the city with projected messages, like UK approval ratings and, more notably, a red baseball cap with the USS John McCain on it.
And the "Trump baby" blimp will also be back, flying over parliament.
In what is likely to be the most unconventional state visit in recent memory, Trump has already waded deep into British domestic politics.
In an interview with the Sunday Times, he said Britain should refuse to pay its 50 billion dollar divorce bill - Walk away from Brexit talks if Brussels won't give it what it wants - And that arch-Brexiteer Nigel Farage should be the one to conduct those talks.
Trump also weighed in on who Britain's next prime minister should be.
He likes former foreign minister Boris Johnson, who favors Britain leaving the EU in October with or without a deal.
Trump is also expected to use the trip to demand a tougher British stance on China and telecoms giant Huawei.
Britain prides itself on its so-called special relationship with the United States.
Theresa May was the first foreign leader to visit Trump after his inauguration.
But some Brits see the leader as volatile and hostile to their values on immigration and the treatment of women.