A crowdfunded campaign to launch a giant "Trump Baby" blimp into the skies above central London during US President Donald Trump's upcoming visit to the UK, has smashed fundraising targets.

More than 10,301 pounds ($18,427) has been crowdfunded to get the six metre-high, orange inflatable figure up in the air — smashing the original target of $9,000.

The anti-Trump activists behind the campaign are determined to have their sentiments known by the President himself.

Activists hope to fly the blimp over Parliament Square during Mr Trump's visit. ( Supplied: Leo Murray )

"Donald Trump is a big, angry baby with a fragile ego and tiny hands," the crowdfunding page said.

Activists called Mr Trump a "racist demagogue who is a danger to women, immigrants and minorities and a mortal threat to world peace".

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"Moral outrage is water off a duck's back to Trump. But he really seems to hate it when people make fun of him," they said.

"So when Trump visits the UK on Friday the 13th of July this year, we want to make sure he knows that all of Britain is looking down on him and laughing at him."

However, despite the successful fundraising, London Mayor Sadiq Khan has not given activists permission to fly the blimp as it does not qualify as a protest.

But that has not done much to deter them.

More than 7,000 people have signed a petition, titled Let Baby Trump Fly, calling for the blimp to be allowed to fly over Parliament Square during Mr Trump's visit.

There is even talk of hiring a barge to "guarantee that Trump Baby will fly".

"If we are unable to obtain consent to fly Trump Baby from Parliament Square Gardens, we may need to be creative about finding a suitable alternative location," the protesters say.