Anti-Brexit protesters are to march through central London just days before the Britain's new prime minister is announced.
The 'No to Boris, Yes to Europe' demonstration will process from Mayfair's Park Lane to Parliament Square in Westminster from midday on Saturday.
Boris Johnson, widely expected to be named as winner of the Tory leadership contest on Tuesday, is the protest's main political target.
Throughout his leadership campaign bid, Mr Johnson has pledged to lead the UK out of the EU by the October 31 deadline.
An inflatable 'Boris Blimp' will be launched in Parliament Square from 10am on Saturday, ahead of the main demonstration.
The balloon is inspired by the 'Baby Trump' blimp that mocked US president Donald Trump during his visit to the UK last month.
March for Change, a network pro-Europe grassroots groups organising Saturday's demonstration, said the Boris Blimp featured Mr Johnson's "faux-dishevelled hairstyle and the mismatched running gear favoured for his doorstep photo-ops".
The inflatable version of the former foreign secretary will be wearing a T-shirt emblazoned with a red bus and the figure £350m – a reference to Mr Johnson's Brexit referendum promise of clawing back funding for the NHS.
Tom Brufatto, chair of Britain for Europe, one of the grassroots organisations behind Saturday's protest, said: "The Boris Blimp might seem a bit of lighthearted fun but it carries a serious message.
"We will not allow Boris Johnson to float into Number 10 on a favourable tailwind, or have the summer off, unopposed, after so much hot air on Brexit, with so much at stake.
"His utter contempt for British democracy, signalled by his willingness to shut down Parliament, must be challenged as he's about to receive the keys to Number 10."
A 9ft tall representation of Brexit Party leader Nigel Farage will also be towering over protesters at the demonstration.
Organised by anti-Brexit campaign organisation Best for Britain, a stilt-walker wearing a custom-built head of Mr Farage will hold two puppet versions of Mr Johnson and his Tory leadership rival Jeremy Hunt.
Best for Britain chief executive Naomi Smith said: "Brexit exposed some of the big problems successive governments have been neglecting for decades.
"Farage wants to use those problems to promote his own agenda and he has both Conservative leadership hopefuls in tow. He's pulling the strings.
"Brexit isn't the solution - and we will fight to stop it. But once we've got that obstacle out of the way, we all need to make sure things don't go back to how they were.
"We need change and that's what this demonstration is about."
An independent 'Left Bloc rally', organised by campaigners from Another Europe is Possible, will also be held before Saturday's main demonstration.
Protesters will gather at Stanhope Gate in Mayfair from 11am to hear speeches from Labour MPs including Clive Lewis and Kate Osamor, alongside MEPs, trade unionists and other left-wing campaigners.
Ana Oppenheim, from Another Europe is Possible, said: "This demonstration is taking place at the start of a seriously dangerous moment in British history.
"Brexit is the British franchise of a global rise in right-wing nationalism, from Italy to the United States, and Boris Johnson is our Donald Trump.
"The left and the labour movement must unite to fight against the tide and stand up for the rights of migrants and the oppressed."
Speakers, including commentator Owen Jones and singer-songwriter Billy Bragg, are also due to address protesters in Parliament Square from 2.30pm on Saturday at the end of the demonstration.
