Thousands of people descended on London's Trafalgar Square to call for urgent action on climate change.
The free event organised by the Stop Climate Chaos Coalition took place on the eve of global climate talks in Nairobi.
At the rally, activists urged the Government to negotiate an international deal to keep global warming to less than two degrees centigrade, and to lead by example by introducing a Climate Change Bill into the Queen's speech which delivers annual cuts in UK carbon dioxide emissions.
They also want to see Britain assist developing countries to adapt to climate change.
Ashok Sinha, director of the coalition, which is made up of a number of environmental and development organisations said the event was to encourage support and put pressure on the Government to act.
He said: "The UK Government can made an important case at the talks next week. There is a danger threshold: if we breach it, it could be catastrophic.
"The Government have enough evidence themselves that something needs to be done.
"We can make the argument about how many people could suffer. We can make the argument on the impact on the eco-systems, we can make the argument about peace and security - 100 million people could become climate change refugees but if any other arguments still need to be made we could say that economically it is of fundamental importance. We can save much money in the long run if we deal with this problem now. That argument is also a powerful one.
"In terms of raising awareness publicly we are asking people not just to turn up today, we are asking them to sign up to our I Count campaign, to show they believe that changes need to be made."
This week, the Stern Report focused on the financial costs of climate change and the Government announced that it accepts urgent action is needed.
