Pope Francis was absent when the Catholic Church traditionally began the Lenten season on Ash Wednesday.
One of the cardinals led a short procession between two churches on the Aventine Hill in Rome in the Pope's place and delivered a sermon.
We feel deep communion with him at this moment, said Cardinal Angelo De Donatis.
The 88-year-old Pope has been struggling with double-sided pneumonia for three weeks. On two occasions recently, he has been forced to be on a respirator. The night before Wednesday was calmer and Francis slept with an oxygen mask.
The condition is stable, currently not critical, according to the Vatican in what is described as a cautious prognosis.
The Pope was more physically active on Wednesday when he spent the day working while sitting in an armchair. Among other things, he talked on the phone with Father Gabriel Romanelli in Gaza.
