They were taking part in a vigil calling on the Government to impose a moratorium on building coal-fired power stations in the UK until all carbon emissions can be captured and stored from the outset.

The demonstration was organised by the Stop Climate Chaos coalition, an umbrella body of more than 100 organisations with a combined supporter base of 11 million.

Other organisations represented included Greenpeace, Oxfam, RSPB, WWF and the World Development Movement.

Addressing the demonstrators, Ed Miliband said that he hoped Prime Minister Gordon Brown would attend in person the crucial UN climate summit in Copenhagen in December.

Christian Aid climate campaigner Laura Trevelyan said: "Coal is responsible for more than half of the carbon emitted historically.

"The impacts of climate change caused by those emissions - floods, drought, and a higher incidence of disease - are already killing people in the developing world. Such communities have the least responsibility for global warming."

She said Miliband's current proposals would still allow new coal plants to pump 75 per cent of their emissions into the atmosphere.

"We need a cast iron guarantee that no new high-emission power plants will be built in the UK," she continued.

"Anything less would be seen by the developing world as another signal that rich countries like the UK are not serious about tackling climate change.

"Anything that helps support that impression will seriously damage the chances of the international community arriving at a meaningful climate deal in Copenhagen."