A girl of about 14 is sobbing, eyes closed. In front of her, a large woman is teetering around, crashing into others in the crowd, while a man in a white shirt and nametag holds on to her forehead, screaming into her ear. On the stage, a middle-aged man lies weeping, more wailing actually, while a young woman in a smart uniform crouches over him, dabbing at his eyes with toilet tissue. All this is filtered through an intense babble – the sound of 100 primal pleas to God. Welcome to Friday night at the Finsbury Park Rainbow, which for just over 10 years has been owned by the Universal Church of the Kingdom of God (UCKG).

The Rainbow was once a grand old cinema, the Astoria, built in the 1920s and decorated Alhambra style. Later, as a music venue, it was the scene of teen hysteria, as Osmonds fans rioted outside in a desperate, ticketless attempt to bear witness to their gleaming-toothed gods. Today, a similar fervour surrounds the Rainbow, but it is pastors, not pop stars, that the crowds flock to see.

UCKG is one of the world's fastest-growing new religious movements. Its roots and power base are in Brazil, where it was founded in 1977 by 'Bishop' Edir Macedo Bezzera, a former lottery salesman. Since then, Macedo has built a substantial media empire, and even holds significant political power. 18 members of Brazil's lower house declare themselves as members of UCKG, with many other politicians jealously vying for its support. In the UK, the church currently operates from 15 premises, circulates a free newspaper called City News, and fully owns Liberty Radio, a London-based station that broadcasts a combination of saccharine devotional songs and evangelical messages.

Earlier this summer, billboards started appearing, all over London bearing the word "Signs' in flame-orange letters on a black background. A sign that says 'Signs' might seem more of a postmodern tease than a genuine announcement. But it turned out that Signs was a mass evangelical meeting to be held in West Ham United's Upton Park stadium.

So on a hot July day, tens of thousands of people packed into the stands to pray for God to solve their problems. Actually, UCKG adherents don't really 'pray'. They don't even implore. They command. Where most Christians are just glad God hasn't decided to smite the lot of us on any given day, UCKG'ers set out a kind of spiritual shopping list of demands: healing, affluence, peace of mind. At one point at Upton Park, the preacher even challenged God to show himself and perform a miracle – something that is explicitly forbidden in the Bible.

Mind you, there is one verse of the Bible that is very strictly upheld. According to UCKG literature, Genesis 14:18-20 is key: "And Melchizdek King of Salem brought forth bread and wine: and he was the priest of the most high God. And he blessed him and said 'Blessed be Abram of the most high God, possessor of heaven and Earth: which hath delivered thine enemies into thy hands.' And he gave him tithes of all."

Tithes are central to the UCKG's doctrines. It is expected that each member give 10 per cent of his total pre-tax income to the church. This means 10 per cent of all income: not just salary, but child benefit, pension, student grants, loans, interest, everything. Even people on the dole are expected to give up 10 per cent.

Not to tithe is akin to robbing God (it's in Malachi 3:8,9, apparently). But the church literature also makes it clear that even those who do donate the statutory tenth are not guaranteed a place in Heaven. Always read the small print.

This system of income collection has been massively successful for the UCKG. In the 2004/2005 financial year, the church income from donations was a pretty impressive £5,369,807, of which about 80% came direct from donations at services (both tithes and gifts).

Money clearly oils the squeaky God wheel, as is evidenced at the start of Friday night 'Spiritual Cleansing' at the Rainbow. At the very beginning of the service, the preacher tells people to hand over their envelopes, which contain money and prayers.

And then, after just a few minutes of an identifiable Christian service, comes the transformation. A crowd of people who 10 minutes earlier had been happily chatting and laughing suddenly morph into a juddering, screaming, wailing, girating horde. This is mass hysteria inspired by the exhortation of the preacher that misery, suffering and badness are all caused by demons. Demons who can be exorcised through a combination of prayer, and perhaps more importantly for the UCKG, payola.