A large police presence has been building up outside the palace in Oxfordshire and BBC News correspondent Duncan Kennedy said about 500-600 protesters bearing placards were demonstrating before Mr Trump's arrival.
The state visit was never actually scheduled - there has been fierce opposition to it in many quarters of Britain - and this trip is part of a working visit instead, meaning among other things that Mr. Trump will overnight at Winfield House, the USA ambassador's handsome residence near Regents Park, rather than at Buckingham Palace or Windsor Castle as a guest of the queen.
"I mean, the list goes on", he said. But instead of attending business in London, which is what most heads of state who arrive in the United Kingdom do, Trump will head to a business sector gala at Blenheim Palace, the birthplace of Winston Churchill which is outside the city.
Some British citizens are even more distressed about Trump than they are about the U.K.'s own feverish political turmoil.
After four days in the UK-including two days at his properties in Scotland-he will head to Helsinki to meet with Russian President Vladimir Putin.
It has also complained that 300 officers working during the visit will have to sleep on camp beds in a gymnasium that has no hot water and restricted access to hot food.
Trump flew in after a fraught North Atlantic Treaty Organisation summit in Brussels where he piled pressure on allies to double their defense spending.
Opponents to the controversial USA leader have threatened mass protests but Mr Trump is expected to avoid areas such as central London where demonstrators are likely to gather.
A 19-foot-tall orange balloon, shaped to look like a baby version of President Trump, will be flown in central London.
Asked if the president will visit Menie, Eric said: "It's still to be decided".
Trump "has proudly sparked passionate often divisive emotions about things, I think Americans living here to eager to turn out and express their views", he said.
The leaders are also expected to discuss Russia, Brexit and the Middle East.
However, Mr Trump declared the British people like him a lot, as he began his first official visit to the country.
Protests are expected against Trump, whose policies - including a travel ban on a number of predominantly Muslim countries, detention of child migrants on the US-Mexico border and imposition of tariffs on European Union steel and aluminium exports - have all been criticised by the UK. But one woman said Trump had given a "short speech" which she described as "lovely".
A number of protests have been planned for the president's stay in the country, with the largest march due to be held in London on Friday.
The presidential motorcade also includes two identical limousines, one of which is used as a decoy and backup should any problems arise.
President Donald Trump will get the red-carpet treatment on his brief visit to Britain beginning today: military bands at a gala dinner, lunch with the prime minister at her country residence and tea with the queen at Windsor Castle before flying off to one of his golf clubs in Scotland.