Queen Elizabeth II has formally accepted Theresa May's resignation from the post and appointed Johnson, former London mayor and UK foreign secretary, as new head of the UK cabinet.
The rally in central London was attended by supporters of the opposition Labour party, environmental activists and anti-Brexit protesters from all walks of life.
A crowd of mostly young people was seen holding up placards that read "Not My Government, Not My Prime Minister," "No to Islamophobia, No to War" and "Tories Kill!"
A source in the capital’s Metropolitan Police told Sputnik over the phone that they had no information on arrests or violations of public order. Dozens of police officers were sent to uphold the law at the protest venue