LONDON — The protesters began assembling even before Air Force One touched down outside London on Thursday. They inflated a giant orange balloon depicting President Trump as a pouting baby, wearing a diaper and wielding a smartphone. They jeered as the helicopter Marine One took off to ferry Mr. Trump to a black-tie gala dinner outside Oxford with Prime Minister Theresa May.

Outside the dinner, at Blenheim Palace, where Churchill was born, a heavy police presence kept around 1,000 chanting demonstrators far away as Mr. Trump and Mrs. May, along with their spouses, were serenaded by trumpeters and other members of a military band.

Mr. Trump’s long-delayed first visit to Britain as president was supposed to cement ties with America’s closest ally, reassure a nation anxious over the chaotic process of exiting the European Union and give Mr. Trump a relaxing weekend in Scotland, where his mother was born and where he owns two golf courses.

But British protesters saw in the visit a chance to register their displeasure at Mr. Trump’s policies. The demonstrations will culminate in a march on Friday — with the baby balloon flying overhead — that is expected to be one of the nation’s largest rallies since the 2003 protests against the American- and British-led invasion of Iraq.