The Vice President of Christian development agency Tearfund challenged the government on Saturday to back its strong statements on the need to tackle climate change with hard cash and policies.

Addressing an estimated 25,000 people at the 'I Count' climate change rally in Trafalgar Square, London, the Rt. Rev James Jones, Bishop of Liverpool, said he had witnessed first hand the impact of climate change on poor communities in the developing world.

Bishop Jones said it was "fantastic" that government ministers had this week "lent their weight" to the Stern report on climate change, which warned that global warming could shrink the global economy by 20 per cent.

He added, however, "Sadly 'lending' is the word. The government give it one day then take it back the next.

Over the next 15 years the government will spend £45 billion on new school buildings. He challenged the government to build each of these to the "highest specification of sustainability and to the lowest possible emissions of carbon".

"This is the environment in which our young people should learn about the world. Let the Government put our money where their mouths are!"

Bishop James Jones continued: "Too much is at stake. How can we claim to be empowering Africa to a new future through aid, trade and debt relief, if through our own profligacy we wreck the climate and ruin their harvests?"

The 'I Count' event - the UK's largest ever climate change event - is part of an unprecedented and rapidly growing public campaign which counts among its many supporters the Women's Institute to the band Razorlight and hundreds of Tearfund supporters were among the crowds.

Ruth Weston, a 21-year-old student boarded a bus in Exeter with a group of friends at 6.30 am in order to reach Trafalgar Square for the start of the rally. "We've come as a public demonstration of what we care about the most," she said on the day.

"It's written all over God's word that we are meant to be stewards in looking after creation. I have bought energy-saving light bulbs and I try to walk or use as much public transport as I can."

Sixteen-year-old student, Joshua Amesbury from Chichester, said: "Climate is an important issue which we are facing and if we don't do something about it now, the world is going to be spoilt for people in the future, and we don't want to let that happen."

Brit rock band Razorlight headlined the stage at the rally alongside KT Tunstall. The band's lead singer Johnny Borrell told the gathered thousands: "Today is all about showing that together we make a difference, together we can send a message, together we can stop climate chaos and together we count."

'I Count' supporters turned up in droves for the rally with many coming by environmentally friendly means. Some came by bike, one Tearfund supporter came by canoe from Oxford. Those who could came on foot and even in bio fuel green taxis to highlight what many believe to be the greatest ever man-made threat. Surfers Against Sewage arrived wearing wetsuits and carrying surfboards.

Actress Miranda Richardson and singer KT Tunstall were among those who demanded that Tony Blair take real political action in response to the millions of people worldwide already cutting their personal carbon counts.

Bishop James Jones said he was thrilled to see many Christians among the estimated 25,000 people. "I've been to Africa and India with Tearfund and seen the ruined harvests and the rice fields where children drowned in the floods.

"In Africa there's a saying that we have 'borrowed the present from our children'. That's typical African generosity and English understatement. We have not borrowed the present - we have stolen it from them and we are stealing their future. That is not just a crime against humanity; it is an offence to God."

Ashok Sinha, Director of Stop Climate Chaos, said: "Tony Blair must listen to the urgent demands of over 25,000 people in Trafalgar Square today and millions across the UK. Now he needs to make his government's actions count too, by introducing a new law to reduce carbon emissions year-on-year. He must take a lead on the global stage to keep global warming below a two degree Celsius increase. If we all come together we can stop climate chaos - together we are irresistible."

Andy Atkins, Tearfund's Advocacy Director, described the 'I Count' rally as 'thrilling', but said there remained a challenge to both Christians and churches to cut their own emissions of harmful greenhouse gases and to keep pressure on governments to tackle the issue globally.

"There are tens of thousands of churches of all denominations in the UK. Imagine if they all set out to cut their emissions by 3 per cent a year, which is what we are asking of the government. That would set an exciting example of integrity and determination that the rest of the country could follow."