Marchers converged on Westminster from different sites across the capital.
Environmental protest groups have claimed they will band together this autumn to bring Westminster to a standstill and drive the police to “breaking point”.
Hundreds of activists staged a sit-in outside Parliament on Saturday, in what one organiser described as “normal people dipping a toe in” before widespread civil disobedience on October 1.
Groups including Insulate Britain, Stop the War, Just Stop Oil, and former Labour leader Jeremy Corbyn’s Peace and Justice Project want to force the Government to reduce energy bills while banning the extraction of fossil fuels.
“The idea is that we’re getting normal people dipping a toe in civil disobedience, and sitting in the road is really the first step to not doing what you’re told,” Gabby Ditton, 28, from Norwich, an organiser of Saturday’s protest said.
“And then hopefully everyone will come back in October where the plan is to get thousands of people arrested.
“When they ask you to move you say, ‘I’m very sorry officer, I can’t do that, unless you give me my demand.’
“Effectively, you get so many people arrested that the police reach breaking point, they can’t cope anymore.”
Several activists in Westminster seemed to have been drawn from the ranks of Extinction Rebellion, flying the same XR flags that were seen when London ground to a halt in 2019.
Russ Peterson, an Extinction Rebellion member who travelled down from Northampton, said he was marching for his children and grandchildren.
“Look at what we’re doing to the planet and what they’re going to have to live with,” he said.
“We had a small glimpse of it Monday and Tuesday this week when it was 40C, and you think, that was two days.
“Can you imagine that for a week or two weeks? It would be unbearable, and that’s what’s going to happen unless we change things drastically.”
Marchers converged on Westminster from different sites across the capital, shouting slogans against fossil fuels as they moved through the central London traffic.
Cheers went up when two groups met at the corner of Whitehall Place on the way to Parliament Square, which has been stripped of its grass by this week’s heatwave.
Among the climate change protesters was Nelly, a makeshift white elephant and veteran of a dozen marches, which theatre designer Michael Taylor created out of plastazote, nylon and bamboo.
The costume was held aloft by two activists, clad in thick padded trousers, for over an hour as they headed towards Parliament in temperatures of around 24C.
A third man, wearing camouflage gear and what appeared to be a bus conductors’ hat, held Nelly’s trunk with a piece of string and occasionally bent down to check on the occupants.
“It’s hot and it’s heavy – it’s ok for about two hours, and that’s really when you long to hand it over to someone else,” Mr Taylor said.