LONDON — The most anticipated installment of Britain’s “Stop Trump” protests — a giant orange balloon of President Trump depicted as a pouting baby in a diaper and holding a smartphone — took flight on Friday from Parliament Square in London.
As if they were waiting for a rocket launch, dozens of excited people — including activists, tourists, children and bystanders diverted from their commutes — gathered around the 19-foot balloon and counted down from 10 before it was released into the air.
“This is a victory,” said Leo Murray, an activist and the creator of the balloon. “People love it, he hates it, and it’s driven him out of London.”
Mr. Murray and other activists behind the inflatable “Trump Baby” have called the balloon a “symbol of resistance,” aimed at sending Mr. Trump a clear message that he is not welcome in Britain.
“The only way to get through to him is to get down to his level and talk in a language he understands — one of personal insults,” Mr. Murray has said.
Thousands of demonstrators are preparing for a national rally in London at 2 p.m. to protest Mr. Trump’s policies.
“He mocks and insults anyone who doesn’t support him,” said Adam Cottrell, one of the activists behind the balloon protest, “so now he can see what it feels like.”
Not everyone was enthusiastic about the balloon. Lucy Lawson, an American expatriate, came to see it because it was close to her work, but while she opposes Mr. Trump’s policies, she said the protest was childish.
“Why are people going down to his level? Why are they being so childish?” she said. “It’s because of his childlike leadership that we are in this mess.”
The working visit was already upended by The Sun’s publication of an interview with Mr. Trump in which he gave a harsh assessment of Prime Minister Theresa May’s “Brexit” strategy and praised Boris Johnson, her Conservative Party rival, as a potentially great prime minister. The article was published just as Mr. Trump and Mrs. May were wrapping up dinner at Blenheim Palace.
Mr. Trump spent the night at Winfield House, the American ambassador’s residence in London, where protesters said they had aimed to keep him awake by banging pots and pans and playing recordings of crying children separated from their parents at the Mexican border.
On Friday morning, he and his wife, Melania, headed to the Royal Military Academy Sandhurst for a military display, and were to travel later to Chequers, the prime minister’s country residence, for talks with Mrs. May on a range of foreign policy issues.
Ms. Lawson asked one of the organizers why they had launched the balloon when they knew Mr. Trump would not be in London.
“It’s going to swamp his Twitter feed,” Mr. Cottrell said. “There’s no way he doesn’t see this.”