CARDINAL Cormac MurphyO'Connor added his voice to calls for justice in trade between wealthy and poor countries.
The Archbishop of Westminster was among 12,000 people who descended on Parliament to lobby MPs on the issue of fair trade.
They wanted changes in the rules that govern international trade to make them fairer to the world's poor.
They were met by hundreds of MPs, including Prime Minister Tony Blair and Conservative Party leader Iain Duncan Smith.
Afterwards, Cardinal Murphy-O'Connor celebrated Mass at Westminster Cathedral, which was attended by many of the campaigners.
"This was lobbying with a purpose — for trade justice in our world today," the Cardinal told them.
"More and more, the Catholic community should take part in working for justice in our world. Our prayers, our commitment, our work through this kind of justice is crucial to fulfilling the Lord's purpose."
Cafod director Julian Filochowski said that the event demonstrated the massive grass roots interest in helping the world's poor.