Pope Francis has said his predecessor Pope Benedict XVI is "very sick" and asked the faithful to pray for the retired pontiff so God will comfort him "to the very end".
Francis did not elaborate on Benedict's condition as he made the surprise appeal at the end of his general audience.
But the Vatican has said the 95-year-old's condition had suddenly "worsened due to age" in recent hours.
Benedict became the first pope in some 600 years to resign in 2013, with the former pontiff taking the title pope emeritus.
Since then, he has been living in the Vatican.
"I'd like to ask all of you for a special prayer for Emeritus Pope Benedict, who, in silence, is sustaining the church," Pope Francis said in his remarks.
"I remind you that he is very sick.
"Let's ask the Lord to comfort him and sustain him in this testimony of love to the church to the very end."
Pope Francis went to visit Benedict after the service.
A Vatican spokesperson said he is receiving constant medical care and his condition is "under control".
Benedict has become increasingly frail in recent years after dedicating his post-papacy life to prayer and meditation.
Some cardinals and canon lawyers have questioned Benedict's decisions on retirement, including his decision to continue wearing the white cassock of the papacy.
Another point of contention has been Benedict refusing to revert back to his birth name Joseph Ratzinger.
Critics say those choices and Benedict's continued presence in the Vatican have been confusing for Catholics and threaten the unity of the Church.
Traditionalists have been able to use Benedict as a conservative point of reference when they've been unhappy with Francis's decisions, they say.
Benedict resigned in 2013 due to his "deteriorating" health.
In his resignation letter he said: "After having repeatedly examined my conscience before God, I have come to the certainty that my strengths, due to an advanced age, are no longer suited to an adequate exercise of the Petrine ministry."
Pope Francis has asked for prayers for Benedict days after he called for an end to the "senseless" war in Ukraine.
In his 10th Christmas blessing, he said people should look beyond the "shallow holiday glitter" and help the homeless, immigrants, refugees and the poor.