The first lady wore a bodycon beige dress, cinched with an orange skinny belt, accessorized with beige stilettos.
"It's important for us to send a powerful signal to those who are resisting and campaigning in America that we are standing in solidarity with them", Rehman said.
Protesters have been encouraged to bring pots, drums and vuvuzelas.
Labour Party legislator Paul Flynn, who has criticized Mr. Trump in Parliament, says the US leader has outraged Britons - and people around the world - with his harsh treatment of immigrants. That includes this week's resignations of members of the Cabinet and Parliament over May's proposals to keep some agreements intact after leaving the European Union next year.
"Be aware of your surroundings (and) exercise caution if unexpectedly in the vicinity of large gatherings that may become violent", it added.
But Donald Trump is not just any leader.
Trump confidant Chris Ruddy believes the President, who is due to fly into the United Kingdom today, will not react how people think he will to the balloon.
The demonstration is organised by 'Scotland United Against Trump.' It is to be noted that the US President is also expected to visit Scotland, where he owns a number of golf resorts, the day after his visit to London, read a statement by the Foreign Press Association.More news: Lakers GM Reveals How LeBron James Informed Team Of Free-Agent Decision
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Its appearance in central London was approved by the Greater London Authority.
Opponents to the controversial USA leader have threatened mass protests but Mr Trump is expected to avoid areas such as central London where demonstrators are likely to gather. Vince Cable, the leader of the pro-EU Liberal Democrats, said, "Most of us don't think he is particularly good judge on Brexit or anything else".
It will fly in Parliament Square between 9.30am and 11.30am tomorrow morning, before following Trump up to Scotland as the President continues his tour.
In London, we speak with Sheila Menon, social justice activist and one of the organizers behind the Trump baby blimp. "It's about lifting the spirits of the nation and it's already doing that, you know, it's just putting smiles on the faces of people who had started to despair about the state of politics".
The U.S. Embassy has warned U.S. citizens to keep a low profile during the visit because of the protests, the sort of warning usually issued in more volatile locations.
Melania Trump has heard about the plans and isn't excited about her husband's presidency being see as a 'joke'.
Organisers have also applied to fly it over his Turnberry golf course in South Ayrshire on Saturday, where they hope it will be seen by the president.
Protesters are set to descend on the Regent's Park residence, with plans to blare out recordings of children crying for their parents at U.S. detention centres. When Trump canceled his plans a few weeks before the big day, some - like London Mayor Sadiq Khan - speculated that the expected opposition scared off Trump and was happy about it.