Air Force One lands at Stansted Airport.
The Metropolitan Police will be responsible for the protection and security of the U.S. president and will also try to ensure lawful protests can be staged.
Friday night he will stay at the US Ambassador official residence in Regent's Park, London, which has been surrounded by a ring of steel of new security fences and road barriers to prevent a terror attack.
The largest demonstration is expected in central London on Friday where a giant "Trump baby" balloon will fly close to Parliament after being given the go-ahead by London mayor Sadiq Khan.
"Yes, there might be protests", he said.
The organisers behind the blimp however, said they were just "poking fun" and "hope Trump sees it".
Trump and his wife, First Lady Melania, will land at London's Stanstead airport after attending a North Atlantic Treaty Organisation summit in Brussels.
On reaching the residence the pair walked across the lawn hand-in-hand. In London, we speak with Sheila Menon, social justice activist and one of the organizers behind the Trump baby blimp.
"The President is not avoiding anything", he said.
Many Britons are opposed to Trump's trip which is not the full state visit he was originally promised with the pomp and ceremony that entails.
May is trying to unify her deeply divided Conservative Party behind her Brexit plans with some of her own lawmakers openly speaking of a leadership challenge.
LONDON ― President Donald Trump and First Lady Melania Trump touch down in London on Thursday for a working visit, and while his meeting schedule will keep him nearly exclusively outside of the capital, she will be spending some time in town.
He added: "I've been reading a lot about Brexit over the last couple of days, and it seems to be turning a little bit differently, where they're getting at least partially involved back with the European Union", Mr Trump said.
Donald Trump will then visit the Queen with first lady Melania Trump at Windsor Castle before heading off to Theresa May's countryside home Chequers.
Winfield House has one of the largest private gardens in greater London, but it won't be possible to see the "Trump baby" balloon from its grounds.
The U.S. Embassy has warned U.S. citizens to keep a low profile during the visit because of the protests, the sort of warning usually issued in more volatile locations.
In fact, just the opposite, they say.
"I would love to say that I think this is going to be a hugely successful visit but I think it's going to be very hard", he said at a pro-Trump gathering in parliament.
Blimp organizer Leo Murray said it had been created to speak to Trump "in a language that he understands, which is personal insults". I'm 54 years old, we lived through pretty good times.
Trump angered May and many Britons by tweeting inflammatory and unverified videos made by Britain First, an anti-Muslim group whose leaders have been convicted of hate crimes, and by characterizing parts of London and other cities as no-go areas because of a purported Muslim-related crime wave.
It is widely believed that such a schedule has been designed for Donald Trump because of the overwhelming protests that are expected to descend on London during his visit.
On Saturday, Trump supporters will rally near the U.S. embassy in Nine Elms, south London, for an event titled "Welcoming Trump to Britain Rally - Make Britain Great Again", at which a number of right-wing figures are scheduled to speak.
He will then travel to Helsinki for a summit with Russian president Vladimir Putin.