They added that as a whole, polled Britons have a negative view of Trump, but regarding his visit to their country, it can "broadly be described as negative but pragmatic".
In extraordinary comments on British politics, Trump also said former foreign minister Boris Johnson, who resigned this week over the Brexit plan and is seen as a potential challenger to May, would make "a great prime minister".
"The people voted to break it up [Britain's ties with the EU]", Trump said.
"Numerous demonstrations are being planned for July 12 to 14, 2018, surrounding the visit of the President of the United States to the United Kingdom", the U.S. embassy said in the alert on its website.
Trump, who has repeatedly praised Brexit, has expressed enthusiasm for a wide-ranging trade deal with Britain after its European Union departure in March next year. She was hoping to win over critics of her Brexit proposal with the release of a 104-page document outlining details of the plan.
British law requires any petition supported by more than 100,000 people to be considered for parliamentary debate, even though there was no serious attempt to prohibit Mr. Trump from coming to Britain.
A YouGov poll on Wednesday showed 77 percent had an unfavourable opinion of the president and just 50 percent thought his visit should go ahead. "But I believe the people in the UK, Scotland, Ireland - as you know I have property in Ireland, I have property all over - I think that those people, they like me a lot and they agree with me on immigration".
"We are expecting hundreds of thousands of people in London wanting their voices heard because they reject the normalization of a Trump agenda-one that is based around hate and divisiveness", said the Stop Trump Coalition.
Prince William's heartfelt message to England squad after World Cup dreams end
Not only was it his second of the match but it gave England the vital three points they needed to get off to a good start. That felt so good for us here in Russian Federation , and united us more and more, just like it did in our country.
"I'm excited to welcome the President of the United States", says Drew, who's 21.
A high metal fence has been erected around the USA ambassador's central London residence where Trump will spend Thursday night and the embassy has sent out an alert warning Americans in London to keep a low profile in case protests turn violent.
But trip planners may go out of their way to shield Trump from viewing another aspect of the greeting: an oversized balloon depicting the president as an angry baby in a diaper that will be flown from London's Parliament Square during what are expected to be massive gatherings of protesters opposed to Trump's presence.
May said in remarks during a banquet on Thursday with Mr. Trump at Blenheim Palace, Churchill's birthplace.
President Trump and British Prime Minister Theresa May, seen during a North Atlantic Treaty Organisation summit in Brussels on Wednesday, will reconvene this week in the United Kingdom.
While it is not unusual for heads of state to visit Windsor Castle or meet the UK Prime Minister in her country home, it is quite unusual for an American president not to spend any time in London.
Along with trade and security links, 10 Downing Street said the other key areas to be discussed between Prime Minister May and President Trump included Brexit and the Middle East.