Thousands of people are expected to take part in a church service, march and rally in London to draw attention to the exploitation of migrant workers.
The Archbishop of Westminster Cardinal Cormac Murphy-O'Connor will celebrate the special Mass for Migrants at Westminster Cathedral on Monday.
It will be followed by a march to Trafalgar Square for a rally.
There are thought to be 500,000 illegal immigrants in Britain, and organisers say a new underclass is emerging.
Campaigners want the government to introduce policies that would help illegal immigrants gain citizenship.
Notice that word, 'illegal'? It means that these people have no right to be here. They haven't even made an effort to abide by the UK's pretty lax immigration laws. They are no more than criminals. Giving them citizenship would make the British immigration system an even bigger joke than it is at the moment.
Cardinal Murphy-O'Connor told the BBC's Andrew Marr on Sunday that all migrants, illegal or legal, should be welcomed and treated with respect.Even the ones who come here and try to impose their way of life upon us? Even the ones who would turn Murphy-O'Connor's own flock into second class citizens if they got half a chance? Even the ones that commit rape and murder? The ones who are running criminal gangs? Immigrants have no right to be here, and should be treated as the criminals they are if they come here.
Many of the estimated 500,000 illegal immigrants had been in Britain for years, he said.
"And so I think a way's got to be found whereby they can become citizens and have the advantages of that...
"Many of them are married, settled down and so they live in a kind of shadow land. That's not right and it's not fair."
So, if you keep on committing a crime long enough, eventually you should be rewarded for it. Such, it seems, is the logic of the cardinal.