Marches and rallies were held in the capital on July 13 and 14 and photographs showed Trafalgar Square and surrounding streets packed with peaceful protesters.
But Mr Trump’s top diplomat in the UK, Woody Johnson, claimed there were “far, far less”.
Police do not routinely provide estimates of the size of protests.
During a visit to Leeds, Mr Johnson told The Yorkshire Post: “So much has been made of these protesters I didn’t see it and the reports that I get is that it was far, far less than that.
“There were a number of people I think around Trafalgar Square and the reports that I get is that it was far ,far less than that, and there were a number of people around Trafalgar Square and I don’t want to characterise those people and I probably shouldn’t, about exactly what they were protesting.
“Out of a country of 65m, a rather modest response I think.”
The ambassador also said he “tends to agree” with the President’s characterisation during a joint press conference with Theresa May of some of the reporting of the visit as “fake news”
Mr Johnson also accused the media of failing to report the “roaring success” of the visit, during which Mr Trump savaged the Prime Minister’s Brexit policy.
“The news media is trying to make a point on the protests, they’re trying to make a point on the disagreement with Theresa May, I don’t think there’s any disagreement,” he said.