The Vatican says the pope has selected a gold-plated ring believed to have once belonged to Pope Paul VI's secretary.

His coat of arms will be the same one he used as archbishop of Buenos Aires, but with a bishop's miter and gold and silver keys - both papal symbols - added.

Tuesday's ceremony will include more than 130 government delegations, and representatives from Jewish, Muslim, Buddhist, Sikh, and Jain faiths.

The simplified mass is expected to last for two hours.

On Monday, the pope held a private audience with the president of his native Argentina, Cristina Fernandez.

The two had disagreed over Fernandez's implementation of policies such as mandatory sex education and free contraception.