The cardinal leading the last mass before a conclave to elect a new pope urged his peers Wednesday to choose someone able to protect the Catholic Church's unity and lead a "difficult and complex" point in history.

Cardinals from five continents held a final mass in St Peter's Basilica in the Vatican before shutting themselves away to choose a new leader for the world's 1.4 billion Catholics.

A total of 133 cardinal electors are set to take part in the conclave, the voting process to pick a successor to Pope Francis, who died last month after a 12-year papacy.

"We are here to invoke the help of the Holy Spirit, to implore his light and strength so that the pope elected may be he whom the Church and humanity need at this difficult and complex turning point in history," Giovanni Battista Re, dean of the College of Cardinals, said during the mass.

"This is also a strong call to maintain the unity of the Church... a unity that does not mean uniformity, but a firm and profound communion in diversity."

The cardinal -- who himself is too old to vote -- said it was a choice of "exceptional importance", which required the red-robed prelates to set aside "every personal consideration".

No clear frontrunner has emerged from among the cardinals, who represent a range of........

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