Pope Francis today inducted new Roman Catholic cardinals to the group that will choose his successor, telling them that their high rank was not a privilege but a call to be humble and work for justice.

Pope Francis elevated 20 prelates, many of them from developing countries, at a ceremony in St Peter's Basilica.

It was attended by former Pope Benedict XVI, making only his fifth public appearance at a Church event since his resignation in 2013.

In his homily, Pope Francis said being a cardinal "is not a kind of accessory, a decoration, like an honorary title." He warned against being "puffed up with pride," adding "nor are Church dignitaries immune from this temptation".

"Those called to the service of governance in the Church need to have a strong sense of justice, so that any form of injustice becomes unacceptable, even those which might bring gain to himself or to the Church," said Pope Francis, who has made attention to the poor a plank of his papacy.

Cardinals are the Pope's highest ranking aides in Rome and around the world.

Those under 80 can enter a secret conclave to elect the next pontiff after Pope Francis's death or resignation.

Since his election, Pope Francis has often criticised careerism and privileges in the clergy, setting the example by living in a Vatican guest house instead of the large papal apartments used by his predecessors.

Fifteen of those who received their red "birettas," or hats, are under 80, bringing the total number of "cardinal electors" to 125.

The five older men were honoured for their long service to the Church.

The new electors, whose names were first announced last month, are from Italy, France, Portugal, Ethiopia, New Zealand, Vietnam, Mexico, Myanmar, Thailand, Uruguay, Spain, Panama, Cape Verde and Tonga.

Nine of them come from the developing world.

It was the first time cardinals from Myanmar, Tonga and Cape Verde had been appointed.

Some come from areas affected by violence and social upheaval.

Cardinal Alberto Suarez Inda of Morelia, Mexico, has worked in a region hit by violence related to drug cartels, and Cardinal Francesco Montenegro of Sicily has been in the front line in efforts to help migrants who survive the dangerous crossing from north Africa to the island of Lampedusa.

Thousands more have died.

The appointments, the second time Pope Francis has named cardinals, strengthened the Church in Asia, Africa and Latin America and further shifted its power centre away from the developed world.

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