Large protests are anticipated throughout the trip, including a plan to fly a large balloon over Parliament depicting President Trump as an angry baby.
While Britain regards the United States as its closest ally, Trump's comments on militant attacks in Britain and his retweeting of anti-Muslim videos posted by a leader of a far-right UK group have sparked anger.
Ms Elliott, who is originally from Virginia, said she is hopeful there could be a state visit by Mr Trump next year.
The protest will continue on July 14 when a "carnival of resistance" is scheduled to take place in Edinburgh, coinciding with Donald Trump's visit to the UK.
"It is in the interests of the U.K.to stand up and take a firm moral stance", said Zoe Gardner, a researcher and spokesperson of the Stop Trump Coalition, which is organizing protests this week in cities across the country.
"Exercise caution if unexpectedly in the vicinity of large gatherings that may become violent", the embassy said in a list of advised actions, asking Americans to "keep a low profile" and "be aware of your surroundings".
Blimp organizer Leo Murray said it had been created to speak to Trump "in a language that he understands, which is personal insults".
"I don't think we should roll out the red carpet for somebody that holds such abhorrent views".
A YouGov poll on Wednesday showed 77 percent of Britons had an unfavorable opinion of Trump and just 50 percent thought his visit should go ahead.
Ahead of Trump's arrival today, Mrs May said: "When we leave the European Union we will begin to chart a new course for Britain in the world and our global alliances will be stronger than ever".
May will seek to put the diplomatic tensions behind her when she hosts Trump on Thursday for a black-tie dinner with business leaders at Blenheim Palace, the birthplace of World War II prime minister Winston Churchill.
A barrage of nationwide protests will greet US President Donald Trump's four-day trip to Britain from Thursday, with organisers hoping to stage one of the country's biggest demonstrations in decades following a series of diplomatic spats.
Nearly all of Trump's activities will be outside the city: He'll meet the Queen at Windsor Castle, not Buckingham Palace.