U.S. President Donald Trump slammed London's Mayor Sadiq Khan, a fierce critic of the U.S. president, as he landed in the U.K. for a state visit on Monday.

Trump was met by the U.S. Ambassador to Britain, Woody Johnson, Foreign Secretary Jeremy Hunt and other dignitaries. The president held his salute as he walked through 20 members of the guard that greeted him and was quickly escorted to the Marine One, the presidential helicopter.

Trump tweeted, ".@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 is expected to be greeted with significant protests throughout his time in London.

Trump stirred controversy ahead of his state visit to the United Kingdom this week by throwing his political weight behind Brexit and hand picking a successor for Prime Minister Theresa May who announced she will resign in the coming weeks.

Trump also said Britain should refuse to pay its 39 billion pound EU divorce bill and “walk away” from Brexit talks if Brussels does not give the U.K. what it wants.

In an interview with the Sunday Times newspaper ahead of his state visit to Britain starting Monday, Trump said the next British leader should send arch-Brexiteer Nigel Farage to conduct talks with the EU.

Trump argued Britain must leave the EU this year.

“They've got to get it done,” he said. “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.”

Sadiq Khan, the mayor of London, compared the language used by Trump to that of “the fascists of the 20th century.” Khan published an editorial in the Sunday Observer entitled, "It’s un-British to roll out the red carpet for Donald Trump," in which he chastised Trump for what he sees as the president's embrace of white nationalism and urged outgoing Prime Minister Theresa May to publicly condemn the U.S. president.

Trump repeated his backing for those candidates to succeed Prime Minister Theresa May who have said Britain must leave on the due date of Oct. 31 with or without a deal.

Those candidates include former foreign secretary Boris Johnson, whom Trump praised in an interview with the Sun newspaper on Friday, along with former Brexit minister Dominic Raab and interior minister Sajid Javid.

Trump also said it was a mistake for the Conservatives not to involve Farage, the Brexit Party leader, in negotiations with Brussels after his success in European Parliament elections last month.

“I like Nigel a lot. He has a lot to offer - he is a very smart person,” Trump said. “They won’t bring him in but think how well they would do if they did. They just haven't figured that out yet.”

On the Brexit divorce bill, Trump said: “If I were them, I wouldn’t pay 50 billion dollars. That is a tremendous number.”

Trump also said he would have “to know” veteran Socialist Labour opposition leader Jeremy Corbyn before authorising U.S. intelligence to share its most sensitive secrets with a hard-left government.

He warned British ministers they must be careful not to jeopardize intelligence-sharing by letting Chinese firm Huawei Technologies Co Ltd into Britain’s new 5G mobile phone network.