President Donald Trump arrived in London for a controversial three-day state visit to the United Kingdom Monday, taking a swipe at the city's mayor and injecting himself in the country's leadership race, amid a week-long European tour to mark the 75th Anniversary of D-Day.

Trump landed at Britain's Stansted airport on Air Force One early Monday morning, with plans to travel into central London later today on the Marine One helicopter, where he will meet several members of the Royal Family and Queen Elizabeth II at Buckingham Palace garden. He will also, however, face a series of protests around the capital, and the broader United Kingdom, ahead of plans to discuss trade, security and climate change with outgoing Prime Minister Theresa May on Tuesday.

"I look forward to being a great friend to the United Kingdom, and am looking very much forward to my visit," Trump said on his official Twitter account Monday. "Landing now!"

Trump, an unpopular U.S. President with the British public and only the third U.S. leader to receive a State visit, has already broken decades of political protocol by expressing a preference for Boris Johnson in the Conservative Party leadership contest to replace outgoing Prime Minister Theresa May, as well as suggesting maverick Euroskeptic MEP Nigel Farage should lead Britain's stalled exit talks with the European Union.

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He also Tweeted harsh criticism for London's Mayor, Sadiq Khan, calling him a "stone cold loser" in twin messages sent as his plane touched down in London. 

Beyond the President's score-settling, however, are important issues of trade in security between the U.S. and its closest European ally, whose 'special relationship' has been fractured of late over disagreements with respect to Britain's EU exit talks and a reported plan to allow blacklisted Huawei Technologies to build some parts of the U.K.'s burgeoning 5G network. 

Trump's meeting with May, her final act as Prime Minister before departing Number 10 on Friday, is also likely to be marked by differences over Brexit and Trump's view that Britain should be prepared to "walk away" from the EU without a bespoke trade deal.

"They've got to get it done," he told Britain's Rupert Murdoch-controlled Sunday Times newspaper. "They have got to get the deal closed. If they don't get what they want, I would walk away."

"If you don't get a fair deal, you walk away," he told the paper.