Waving placards as they drove the red, open-top, double-decker past the Palace of Westminster and St. Paul's Cathedral, the campaigners urged the estimated 200,000 US citizens in Britain to register to vote in the election on November 8.
They stopped off at key American hotspots in the city, including the American School, the City of London financial hub and a popular Philly cheesesteak food truck in Spitalfields, East London.
The event was organized by Avaaz -- a global "people-powered politics" group based in London that provided iPads for US citizens to register for the vote online at the various stops along the bus route. And they tried to attract participation by handing out cookies and blaring American music by Johnny Cash and others from the speakers.
"Millions of Americans live abroad," the group said in a Facebook post. "They could stop Trump from taking over the White House, but only 12 per cent of them vote. This is where we come in."
Around 20 campaigners chanted anti-Trump slogans from the bus and called on US citizens to pick Democratic candidate Hillary Clinton.
Brooklyn native Dana Lazarus-Cass, 55, who has lived in London for 30 years, said she was going to vote Democrat to counter what she considers Trump's isolationist campaign.
"I think Americans who live abroad have a different perspective," she said. "We have a broader view of the world and America's standing in it."
Amanda Jones, 27, from Colorado, also said she was voting for Clinton. Pressed on the former secretary of state's policies, Jones seemed mostly motivated by opposition to Trump.
James Toles, also 27, is traveling through Europe and has already registered to vote.
The New Yorker told CNN, "People who are following Trump, I don't think they even know his politics beyond the (Mexican) border wall, which is absolutely ridiculous."
Avaaz's campaign director was pleased with the result of Wednesday's efforts.
"Today went really well. We had an amazing reception from the general public, waving and cheering at the bus as we drove through London," Meredith Alexander told CNN. "They clearly feel the same way as we do about the threat Trump poses to our world. We had over 50 US expats come on the bus and register to vote."
The Trump campaign did not respond to requests for comment.
The Liberty Cheesesteak truck -- which sells the famous rolls packed with grilled cheese and meat -- is run by JP Teti, who moved to London from his home in Philadelphia eight years ago with his wife.
The upcoming election dominates the conversations for US expats who queue up Liberty Cheesesteak truck in Spitalfields.
Teti said that he isn't voting for either Clinton or Trump, however.
"I think that there are certainly talented people in our country and we could put up better choices for leader of our country," he responded. "Most Americans would like to see that."
Avaaz is planning similar initiatives in Berlin Friday and Mexico City Sunday. In Berlin, activists intend to demolish a 2.5-meter-high (8.2 feet) wall with an image of Trump painted on it.
Invited to comment on the event, Kate Andrews, a spokesperson for Republicans Overseas UK, said registering Americans to vote was an important cause.
"We would encourage all Americans abroad, here in the UK and globally, to register to vote," she said. "Regardless of who one casts their vote for in this election cycle, Republicans Overseas UK remains a voice for all expats who support liberty and constitutional values."
The organization has not hosted any rallies in the UK for Mr. Trump yet and said reliable polling on how much support he has had not been conducted, but Andrews did point to some of the group's upcoming events of its own: "We plan to host some upcoming breakfasts around the later debates."