The event was the largest of about 2,500 demonstrations taking place around the world ahead of a critical UN climate summit in Paris.
Organisers said about 50,000 people were on the route from Hyde Park to Whitehall.
The Paris conference, known as COP21, starts on Monday and will try to craft a long-term deal to limit carbon emissions.
The gathering of 147 heads of state and government is set to be far bigger than the 115 or so who came to Copenhagen in 2009, the last time the world came close to agreeing a long-term deal on climate change.
The London march began at Park Lane at 13:00 GMT and took campaigners past the Houses of Parliament to a rally at Millbank.
Campaigners from groups such as Greenpeace, Friends of the Earth, Oxfam, ActionAid and the World Wildlife Fund marched alongside people from around the UK such as a group protesting against fracking in Lancashire.
Labour leader Jeremy Corbyn, fashion designer Vivienne Westwood, actresses Emma Thompson and Vanessa Redgrave, singers Thom Yorke and Charlotte Church, Green, and the Green MP Caroline Lucas were among those who attended.