Activists had managed to inflate the giant blimp by 9 a.m. on Parliament Square, an hour before the official start of a protest organised by the "Together Against Trump" group in Trafalgar Square, reports Efe news.
The first protesters began convening at around the same time, ready to march from Trafalgar Square towards Parliament and Downing Street, Prime Minister Theresa May's official residence, where Trump would be meeting her for talks.
Also present in the UK capital was a large robot depicting the President sitting on a toilet, apparently engaged in tweeting.
Besides making noises simulating passing wind, the robot, which was wheeled out onto Trafalgar Square on Tuesday morning, also comes out with phrases like "You are fake news" or "I am a very stable genius".
The roads near Parliament were cut off and police officers were monitoring the route along which the activists would march.
The leader of the main opposition Labour party Jeremy Corbyn was expected to address protesters later "in solidarity with those he's attacked in America, around the world and in our own country", he said.
The Scotland Yard has deployed a large police and security operation and has banned protesters from getting near Downing Street.
The protesters had initially planned to protest in front of May's office and residence, but that was ruled out by police.
The organisers have dubbed Tuesday's protest "a carnival of resistance".
Trump and May are expecting to give a press briefing following their meeting and working lunch.
May is set to step down from her post on Friday after her premiership has been marred by a failure to push through a Brexit deal through Parliament.
On Wednesday, Trump's last day of his official visit, the President will attend a ceremony to commemorate the 75th anniversary of the Normandy Landings in the southern English city of Portsmouth, an event that will be attended by Queen Elizabeth II.
( With inputs from IANS )