Setting off from University College London, the red bus drove around the British capital, stopping in the City financial hub and at the Houses of Parliament in an attempt to find US expats to win over.
Around 20 campaigners chanted anti-Trump slogans and handed out leaflets telling US citizens to register to vote and to pick Democratic candidate Hillary Clinton over the Republican choice Trump.
The event was organised by the campaign group Avaaz.
“We know that Americans abroad don’t generally vote but we know that they’re incredibly progressive so this could be the game-changer for this year’s election,” Avaaz campaign director Meredith Alexander said.
Alexander said there were around eight million US citizens living outside the country, which altogether could constitute a “swing state” in the November election.
“It’s absolutely vital that we mobilise this secret swing state so that can defeat Trump,” she said.
Avaaz said there were around 220,000 Americans living in Britain. One of them, Michael Dale, a 70-year-old law lecturer, was on board the anti-Trump bus.
“I am voting for Hillary Clinton. I think there’s no choice,” he said, as the campaigners handed out cookies and Johnny Cash music played in the background.
Avaaz is planning similar campaign initiatives in Mexico and Berlin later this week.