The demonstrators took part in a planned street party in central London before marching to Downing Street.
Johnson took on Wednesday after an audience with Queen Elizabeth, a moment that put an avowed Brexiteer in control of Britain’s exit from the European Union for the first time since the shock 2016 referendum.
Describing his appointment as “dangerous,” some protesters said he could not be trusted and should not be prime minister.
Johnson had a possible foretaste of turmoil ahead when, as he drove to his audience with the queen, Greenpeace protesters tried – but failed – to block the path of his car as his chauffeur drove around them.
Johnson then delivered his first speech as the UK’s 55th Prime Minister outside Downing Street, using the moment to state that Britain would come out of the EU on Oct. 31 “no ifs or buts.”
(Production: Will Russell, Iona Serrapica, Chris Read, Jonathan Shenfield)