An alliance of 120 civic institutions — including churches, trade unions, civil rights campaigners and educational establishments — supported the 4 May demonstration, which took place in London's Trafalgar Square.

The rally was sponsored by the Citizen Organising Foundation through its high-profile Strangers into Citizens campaign. It has persuaded the Liberal Democrats and the Mayor of London, Boris Johnson, among many others, to support its "pathway to citizenship" for at least 450,000 of the 750,000 undocumented people it estimates to be living in Britain.

The Rt Rev Patrick Lynch, auxiliary bishop of Southwark, said the Catholic Church in England and Wales had a long tradition of "standing in solidarity" with workers, especially migrant workers, and he prayed that migrants would not be made "scapegoats" during the economic downturn.

"My first prayer today is that during a time of recession when there will inevitably be job shortages, we as the church will do all we can not to allow migrant workers from within or outside the European Union to become scapegoats and targets of people's frustration with the economy," he told a Mass for migrant workers held at Westminster Cathedral in central London.

The Mass was being held along with a series of other church services as part of the Strangers into Citizens day of action.

To qualify for the proposed measure, unregistered migrants would have to fulfil conditions including residence of at least four years, followed by a two-year "pathway" period, a clean criminal record, good English and character references.

In his sermon, Bishop Lynch said there was a "clear moral case" that undocumented workers who have lived and worked in this country for five years or more, should be given the opportunity to build a future in Britain and continue to contribute to society.

"You have worked here - your children have been born here and attend school here - you are part of our parishes and our society here and a way should be found so that you can remain here," he told the congregation of different nationalities, some of them in colourful national dress.

In an Anglican service at St Margaret's Church, Westminster, the Bishop of Southwark, the Rt Rev Tom Butler, also backed the campaign.

"What is being called for is no idealistic or wild-eyed demand that all barriers should go down. It is simply recognising that those who have been here for four years or more should be given the opportunity to show that they can be productive and useful citizens," he said.

The bishop added: "This is a call to bring hundreds of thousands of irregular migrants out of the shadows and after a two year probation period, give them leave to remain."

Meanwhile, the anti-immigration lobby group Migration Watch, which receives support from tabloid newspapers and right-wing hardliners, tried to spoil the day of action by claiming that unregistered migrants in Britain would cost the country 'up to £1 million' per family. A civil rights campaigner described this as "malign nonsense".

A recent report from the group, publicised in the Daily Mail, also talked about "migrant stock" in Britain having "doubled since 1991".

Campaigners for migrants' rights are concerned that the UK's increasingly draconian approach to immigration has been fed by racist assumptions and media poison — but say the good news is that broad-based initiatives like Strangers into Citizens are reversing the tide.

"We need to understand that people movements are a fact of modern life," commented Simon Barrow, co-director of the religion and society think-tank Ekklesia. "The problems around migration are caused by injustice and prejudice, not people. Poverty, violence, racism, human rights abuses, economic dislocation and environmental destruction uproot the vulnerable and enable criminality to prosper. These are what we should be targeting, not the vulnerable themselves."

Ekklesia says the Strangers into Citizens campaign is a "vital step in the right direction" toward a just migration policy. It adds that "welcome for the stranger, hospitality for the oppressed and justice for all, regardless of nationality or ethnicity, are moral imperatives deeply rooted in the Christian tradition, alongside those of other faiths and good faith."