Jobs, Justice and Climate, we all shouted, as 35,000 of us, including several hundred from Bristol, joined assorted polar bears, the Four Horsemen of the Apocalypse, a mobile greenhouse which enclosed a model Earth, brass bands, drummers, banners and flags, all on the four-mile walk through London at the weekend ahead of the G20 summit.

This was the Put People First march, a coalition of 150 organisations, including Oxfam and other development groups, environmental organisations, faith groups, diaspora organisations and trade unions, working together to tackle poverty and climate change under the banner of jobs, justice and climate.

We assembled because the "Group of 20" of the world's most powerful countries meets in London for a one-day summit on Thursday. Who are the G20? They are Australia, Brazil, Canada, Argentina, China, France, Germany, India, Indonesia, Italy, Japan, Mexico, Russia, Saudi Arabia, South Africa, South Korea, Turkey, United Kingdom, the United States and the European Union.

It is a very powerful gathering representing 90 per cent of the world's economic output, 80 per cent of world trade and 64 per cent of the world's population.

Top of their agenda will be the global economic crisis and what richer countries need to do to stabilise their financial systems. These include the so-called "global fiscal stimulus", tougher regulation of financial markets, tougher tax avoidance measures and promoting world trade.

Oxfam is particularly concerned about the agenda items that focus the needs of the world's poorest people hard hit by the downturn.

The recession began as a financial crisis, but it's turning into a humanitarian one.

The World Bank – the international institution which offers financial and technical help to developing countries – estimates that an extra 53 million people will fall into poverty because of the global recession.

Although the crisis originated elsewhere, many developing countries are facing a catastrophic loss of income on several fronts. Aid flows are declining as donor governments tighten up. Since January, banks and other financial service firms have already received or been promised at least $8.4 trillion. This equates to more than $1,250 for every man, woman, and child on the planet.

By comparison, the annual cost of ending extreme poverty – the amount which is needed to lift the 1.4 billion people living on less than $1.25 a day above this threshold – is $173 billion.

O xfam is demanding that world leaders should do more than pledge to maintain that aid. It should actually be increased in line with targets agreed at the Gleneagles summit in 2005. That means rich countries should deliver 0.7 per cent of national income as aid by 2013, and push for the cancellation of all illegitimate and unpayable developing country debts, to enable poor countries to invest in and strengthen public provision of essential services.

If aid is not provided, teachers, nurses and police will lose jobs, and children will not go to school. Oxfam is calling for an urgent rescue package. Poor people need help now. They are the ones being hit hardest by the financial crisis they did not, in any way, cause.

Continued and fairer trade is also crucial to poor countries. Africa has been especially affected. We are challenging the misguided belief in unregulated "free" markets that has led to the current crisis, demonstrating that there are effective alternatives.

We are calling for a change in the economic system and concrete actions to benefit people and the environment. G20 must ensure a massive investment in a green new deal to build a green economy, a low-carbon future, based on decent work and fair pay. We can all benefit from this.

Regen SW, the sustainable energy agency, says we can generate 20 per cent of all energy consumed in the South West from renewables by 2020. We can all save money by saving energy.

Climate change is the other side of the coin of the economic crisis. Disastrous climate change will undermine the ability of us all, especially the poorest, to sustain living standards. Three to five planets would be needed if everyone on Earth were to consume as much as an average person in a developed country.

Meanwhile, the poorest consume the least but suffer most from the impacts of climate change. The impact of climate change is already being felt by millions of the world's poorest and most vulnerable – through floods, droughts, increased weather phenomena and rising sea levels.

T he economic costs of inaction are unthinkable. Conservative estimates say doing nothing about climate change will cost the world economy at least five per cent and potentially more than 20 per cent of world economic output – gross domestic product – per year.

World leaders will again be meeting for crucial world climate talks in December 2009 in Copenhagen. Oxfam will continue, as members of the biggest UK group on climate change, the Stop Climate Chaos Coalition to press the UK Government to agree to a deal which delivers substantial, verifiable cuts in greenhouse gases, which will limit temperature increases to well below 2°C.

The march culminated in Hyde Park where, compered by Bristol favourite and actor Tony Robinson, we were entertained by musicians and speakers such as Kumi Naidoo (global co-chair, Global Call to Action Against Poverty), Sharan Burrow (general secretary, International Trade Union Congress) and comedian and campaigner Mark Thomas, who said that he believed the protest marked "the start of a grass-roots movement".

The Put People First event was a fun and positive day and will be continuing to let the G20 leaders know we're still watching them.

Saturday's action, and those staged in cities around the world, will have given them food for thought as they start their meeting. If you could not go on the march, you can still get a ringside seat and take action at the G20 by following the G20Voice project. You can link to 50 expert bloggers, given a high level of access to the G20 summit.

They will let you know exactly how well the leaders are getting on towards the action we want to see from them.

Go to www.g20voice.org and www.putpeoplefirst.org.uk

Roger James is an Oxfam campaigner in the South West.