Thousands of people descended on Westminster on Wednesday to lobby MPs for a better trade deal for the world's poorest countries.

Tony Blair told members of the Trade Justice Movement he was right behind their cause.

"Rich countries can't say they want to help developing countries while at the same time denying them access to markets," the prime minister told campaigners.

The Conservatives and Liberal Democrats were also keen to get behind the campaign, even though the Tories said they did not agree with everything the Trade Justice Movement said.

Shadow international development secretary Caroline Spelman told MPs: "Trade could be the greatest force for poverty reduction in history."

'Keeping up pressure'

Fair trade protesters formed a giant queue along the Thames embankment and across Lambeth Bridge in what the organisers claimed was one of the biggest events of its kind ever held.

Dozens of MPs lined up to meet constituents, who had travelled down to London for the day.

We need your support to tackle poverty and overcome injustice

Thabo Mbeki South African President

Earlier, in Downing Street, Tony Blair told campaigners: "This mass lobby is very timely and at next week's G8 Conference we're hoping for movement on Africa.

"I urge you to keep up pressure on governments, and can assure you that we're building alliances across the world on this issue."

'Ordinary people'

He said the income of developing countries could increase by $150bn a year if there was a 50% reduction in protectionist measures.

The prime minister's spokesman said the issue of EU protectionism was likely to come up at Mr Blair's meeting with France's President Chirac later on Wednesday.

The Trade Justice Movement believes poorer countries are not competing on a level playing field and it wants trade barriers to come down, as well as big companies to pay workers a "living wage".

Campaigners say poor nations are getting a raw deal

South African President Thabo Mbeki said events such as Wednesday's mass lobby "gave us renewed hope" and he said he would be raising it in the South African parliament.

"We need your support to tackle poverty and overcome injustice," he added.

Trade Secretary Patricia Hewitt said she welcomed the rally "wholeheartedly."

"If its success were replicated in every country round the world, imagine the momentum we would have for change," she added.

Britain had fought for developing countries to be put centre-stage at the recent Doha world trade summit, Ms Hewitt said.

And the UK government was campaigning for reform of the EU common agricultural policy, which would allow greater access to the European market for poorer countries' products.

'Great driver of prosperity'

Conservative shadow trade secretary John Whittingdale said: "We congratulate the Trade Justice Movement on bringing this matter to the attention of the public.

"Although we do not agree with all that they propose, we fully share their concern for the plight of the poorest people in the world.

"International trade has been the great driver of prosperity for the past half century.

"International investment is the force pushing globalisation forward, bringing prosperity to many countries in an astonishingly short time."

He said the Tories would continue to push for increased levels of free trade and the breaking down of trade barriers.

The campaigners face opposition from business. Ruth Lea, from the Institute of Directors, said they were right to talk about taking down trade barriers.

"But I think their barriers - wider regulations, social regulations, environmental regulations - could be as damaging to developing countries," she said.

Liberal Democrat international development spokeswoman Jenny Tonge said developing access to trade markets was far more important to poor nations than aid.

"It is high time that the EU and US allowed free and fair trade to tackle the problem of poverty," added Dr Tonge.