Red-hatted cardinals were expected to pick a date Monday for the conclave to elect a new leader of the world's 1.4 billion Catholics, following the death of Pope Francis.

Dozens of so-called "Princes of the Church" from across the world have been gathering at the Vatican since the 88-year-old Argentine pontiff died on April 21.

But so far there are few clues as to who they might choose next.

"I believe that if Francis has been the pope of surprises, this conclave will be too, as it is not at all predictable," Spanish Cardinal Jose Cobo said in an interview published Sunday.

In previous conclaves, "You can see where things might go", he told El Pais newspaper, whereas this time many cardinals hail from beyond Europe and have not even met each other before.

Francis was laid to rest Saturday with a funeral and burial ceremony that drew 400,000 people to St Peter's Square and beyond, including royalty, world leaders and ordinary pilgrims.

Vast crowds also gathered Sunday to view his marble tomb in the Santa Maria Maggiore Basilica in Rome, after the "pope of the poor" opted to be buried outside the Vatican's walls.

With conflicts and diplomatic crises raging around the world, Italian Cardinal Pietro Parolin, who under Francis was secretary of state, the pope's number two, for many, is the favourite to succeed him.

British bookmaker William Hill put him slightly ahead of Filipino Luis Antonio Tagle, the Metropolitan Archbishop emeritus of Manila, followed by Ghana's Cardinal Peter Turkson.

Next in their odds comes Pierbattista Pizzaballa, the Latin Patriarch of Jerusalem, then Guinea's Cardinal Robert Sarah, and Matteo Zuppi, the Archbishop of Bologna.