After the North Atlantic Treaty Organisation summit, Trump will head to England, where Prime Minister Theresa May's government is in turmoil over her plans for exiting the European Union.
Trump heads to Britain just days after several high-level cabinet ministers, including Foreign Secretary Boris Johnson, resigned over Brexit negotiations. The issue is 'Do they have freedom to protest, freedom to assemble and should they be allowed to do so?'
Securing a U.S. -UK deal post-Brexit is something both Trump and May's government have heralded as being a major benefit of leaving the EU.
Mr Khan said he supported the decision taken by the Greater London Authority, adding that it was not for him to be a "censor".
This will be Trump's first visit to England as president. That's not on the schedule, but the president makes his own schedule.
"These are really worrying times and people across the country feel a need to express that we don't want to see that or any of the things he represents being welcomed here".
"In terms of the special relationship - there is a strong bond from history, language, shared western values that is going to continue regardless of the protesters here or in the United States", she said. It's not clear if that means only global visits, or if the organizers plan to bring it to the U.S.at some point.
"I don't think so but people are free to do so it is a free country".
And with a giant "Trump Baby" balloon set to fly in the air over multiple demonstrations, US officials urged Americans to "keep a low profile" in the English capital.
The "Demonstration Alert" issued by the embassy also tells Americans to "exercise caution if unexpectedly in the vicinity of large gatherings that may become violent".
The warning comes with the information that organizers expect a chance that British protests against the president could escalate into violence, and that any Americans blatantly identifying themselves as such during those protests could potentially risk physical harm.