The caricature of Mr Johnson also featured a t-shirt emblazoned with a bus and the figure £350m, after his infamous false claim about the rewards for the NHS from leaving the EU.
Ms Toksvig said she believed “passionately” in the European project, adding: “Some of the enormous political issues we face today – the climate emergency, cybercrime, violence against women and girls – are truly global; we cannot fight them alone, and we cannot fight them outside of Europe.”
The co-founder of the Women’s Equality party said she was “also very worried about what a Boris Johnson premiership means for women in Britain”, with those electing him overwhelming male.
Tom Brufatto, the chairman of Britain for Europe, said: “We will not allow Boris Johnson to float into No 10 on a favourable tailwind, or have the summer off, unopposed, after so much hot air on Brexit, with so much at stake.”
And Naomi Smith, chief executive of the anti-Brexit Best for Britain group, said: “Brexit exposed some of the big problems successive governments have been neglecting for decades.
“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.”
Labour MPs Clive Lewis and Kate Osamor were due to speak, alongside MEPs, trade unionists and other left-wing campaigners.
Mr Johnson, who has vowed a “do or die” Brexit on 31 October is expected to be declared the new Conservative leader on Tuesday and enter No 10 the following day.