One Saturday morning in November, Farhana Yamin took her place in a line of people gathered on Westminster Bridge and – when a signal was given – stepped off the pavement and into the road. As the crowd swelled, cars, black cabs and double-decker buses began to back up in nearby streets.  Similar scenes were unfolding on four other bridges over the Thames in a two­ and-a-half-mile arc spanning the capital’s best-loved landmarks: Big Ben, the London Eye and St Paul’s Cathedral. In a festival atmosphere, protesters danced and passed around cupcakes. Among the thousands of first-time activists: families with kids; a banker; a teacher; a civil servant; grandparents; a vicar. Banners bore the name of the  new movement: Extinction Rebellion.

Starting at 11am this Monday, the group plans to gridlock central London. Volunteers are due to peacefully occupy Parliament Square, Oxford Circus, Marble Arch and Waterloo Bridge. Participants have been asked to bring food and tents. Offshoots in cities across Europe and the US will stage parallel protests. After two previous attempts to get herself arrested, Yamin, an international environmental lawyer and one of the movement’s leading voices, hopes she will soon see the inside of a police cell. “Every now and again I think, ‘What have I done?”‘ says Yamin, who helped midwife the 2015 Paris Agreement to curb greenhouse gas emissions, before growing disillusioned with the results of the 27 years she spent building coalitions to drive formal climate diplomacy. “I just got fed up with the environmental movement selling so much false hope when we’re still trashing the planet.”