In what is set to be one of the largest ever Christian expressions in the public square, thousands will attend simultaneous services celebrating the place of migrants on 4 May before moving to Trafalgar Square to call for large numbers of refused asylum seekers and visa overstayers to be made citizens.
The Strangers Into Citizens 'Day of Action and Celebration' on the Bank Holiday Monday will begin with a series of simultaneous religious services at Westminster Cathedral, Westminster Abbey and Methodist Central Hall in which leaders of the Catholic, Anglican, and free churches will celebrate the place of the immigrant in modern Britain.
The Catholic Archbishop of Westminster, Cardinal Cormac Murphy-O'Connor, the Anglican Bishop of Southwark, Tom Butler, and the New Testament Church of God leader Bishop Eric Brown will be among those leading the services.
The London Citizens community alliance of more than 120 civic institutions - mostly churches, but also including union branches, charities and schools - is behind the rally, which first took place in 2007 with strong backing from church leaders of the Catholic and politicians from all parties.
The Archbishop-designate of Westminster, Vincent Nichols, told the campaign in March 2007: "I give my support to this call for regularisation procedures which will give proper legal recognition to those who steadily contribute to our economy. This is what we owe them."
Since then, the Citizen Organising Foundation's Strangers into Citizens campaign has persuaded the Liberal-Democrat party and the mayor of London, Boris Johnson, to back its call for a "pathway into citizenship" for some 450,000 out of the total of 750,000 estimated UK total.
The Strangers into Citizens Day of Action and Celebration will also bring together Muslims, Chinese, Latin-Americans and people of no faith, in an exuberant day of diversity. Live acts including Asian Dub Foundation, Igbo dancers, Brazilian drummers, and Chinese dragons will entertain the crowds.
Some 3,000 are expected to attend the Mass for Migrant Workers, concelebrated by the three Catholic bishops of London - Cardinal Cormac Murphy-O'Connor of Westminster, Archbishop Kevin McDonald of Southwark, and Bishop Michael McMahon of Brentwood, at Westminster Cathedral.
Another 2,300 will pack the Strangers into Citizens Gospel Service at Methodist Central Hall, hosted by Bishop Eric Brown (NTCG) and Rev. Ermal Kirby (London Methodist).
Meanwhile, 700 people, many of them Zimbabweans, will attend the Strangers into Citizens Service of Prayer and Praise at St Margaret's (Westminster Abbey), where Bishop Tom Butler of Southwark will be the preacher. There will also be services in Soho (Chinese Outreach Church), and for Latin-American evangelicals (Fusion Centre, Elephant & Castle). Muslims meeting at central London mosques will then gather in Trafalgar Square.
The different groups flowing out of the churches will merge with thousands waving Union Jack flags arriving for the Strangers into Citizens rally at midday in Trafalgar Square, at which 20,000 are expected.
Strangers into Citizens, which began in 2006 following a call by Cardinal Murphy-O'Connor at the first Mass for Migrant Workers, has been calling for the Government to implement a one-off regularisation measure that would allow refused asylum seekers and visa overstayers who have put down roots in Britain to become legal.
Campaigners argue that strict conditions - residence of at least four years followed by a two-year 'pathway' period, a clean criminal record, good English, character references, etc. should be applied. The London School of Economics estimates that some 450,000 people would be eligible under these conditions.
The Home Office admits the impossibility of forcibly removing an estimated 750,000 visa overstayers and refused asylum seekers. Current deportations run at 30,000 a year. The Strangers into Citizens campaign argues that resources should instead be concentrated on deporting criminals.
Critics say that an 'earned amnesty' would only encourage more illegal immigrants. But campaigners point to Spain, where levels of illegal immigration have dropped sharply since a one-off regularisation of 700,000 in 2005 was combined with border-tightening measures.
In the US, President Barack Obama is also proposing a similar 'pathway into citizenship' for that country's large 'illegal' population.
Neil Jameson, Strangers Into Citizens coordinator, said: "The current government strategy of imposing heavy fines and document checks on employers as well as deporting families is an inhumane, costly, and complicated way to tackle irregular migration. We propose that those who have been here for four or more years should be admitted to a two-year pathway to full legal rights during which they work legally and demonstrate their contribution to UK economy and society. Combined with the current border-tightening measures, our policy will reduce illegal immigration, and British society will be the winner".
Strangers into Citizens is backed by key NGOs including the Joint Council for the Welfare of Immigrants (JCWI), Immigration Advisory Service (IAS), Jesuit Refugee Service (JRS).
London Citizens is the UK's largest 'broad-based organisation', operating in three connected chapters: TELCO (The East London Communities Organisation), South London Citizens and West London Citizens. Members include churches, mosques, temples, schools, trade union branches, NGOs, charities and associations.
At the London Citizens mayoral assembly in April 2008, Boris Johnson agreed to support Strangers into Citizens, and has since done so publicly. In March this year BBC Panorama examined the proposals.
MPs arriving at Parliament at Westminster tube over the next month have a daily reminder of the Strangers into Citizens campaign in the shape of bright orange paddles at the ticket barriers. The paddles, advertising the Strangers into Citizens logo, will remain in place until the campaign's gathering in Trafalgar Square on 4 May.
For more information see: www.strangersintocitizens.org.uk