The Vatican released the first photograph of Pope Francis since the 88-year-old pontiff was admitted more than a month ago to hospital for pneumonia in both lungs.
The photo shows the pope, bare-headed, seated in a wheelchair and wearing a white robe and purple shawl, in front of a simple altar with a crucifix on the wall.
Taken from behind Pope Francis' right side, his face is not fully visible, but his eyes are open as he looks in a downward direction.
"This morning Pope Francis concelebrated the Holy Mass in the chapel of the apartment on the tenth floor of the Gemelli Polyclinic," the Vatican press office wrote in the photograph's caption.
Concelebration is the joint celebration of mass by senior clerics.
The release of the photo by the Vatican was significant, as the Argentine pontiff has not been seen in public since being admitted to Rome's Gemelli hospital on 14 February for pneumonia, which for weeks doctors considered critical.
He has improved steadily in the past week, however.
In a medical bulletin yesterday, the Vatican said his condition continued to be stable, although he still required therapy to be administered from the hospital.
Doctors not involved in the pope's care have said he is likely to face a long, fraught road to recovery, given his age and other medical conditions.
Pope Francis earlier acknowledged being fragile and "facing a period of trial", as he thanked well-wishers for prayers in a message from hospital, where he has been receiving treatment for pneumonia.
The 88-year-old pope, sent a particularly personal message to the faithful published by the Vatican, as he once again missed delivering the traditional Angelus prayer in person.
"I am sharing these thoughts with you while I am facing a period of trial, and I join with so many brothers and sisters who are sick: fragile, at this time, like me," wrote the pontiff.
"Our bodies are weak but, even like this, nothing can prevent us from loving, praying, giving ourselves, being for each other, in faith, shining signs of hope," he added.
Today is the fifth time the pope's condition had prevented him from personally giving the Angelus prayer, which he usually delivers to the crowd gathered in St Peter's Square following mass.
Last week the Vatican signalled the Argentine pontiff was out of danger after a series of breathing crises earlier in his hospitalisation had sparked fears for his life.
Yesterday, the Vatican said Francis's condition continued to be stable and showing progress, as it has for the past week, but cautioned he still needed therapies to be administered within the hospital.
"The Holy Father still requires hospital medical therapy, motor and respiratory physiotherapy; these therapies, at present, are showing further, gradual improvements," it said.
That message appeared to quash speculation that the pope's progress could signal an imminent release from hospital.
In his message today - which also called for peace in war-torn countries - Francis once again thanked his caretakers and those who have been praying for him.
"How much light shines, in this sense, in hospitals and places of care! How much loving care illuminates the rooms, the corridors, the clinics, the places where the humblest services are performed!" he said.
Francis is being cared for on the 10th floor of the hospital, home to a special papal suite including a chapel.
Pope Francis, elected pope in 2013, celebrated his 12th anniversary from hospital on Thursday.
At the entrance of the hospital this morning, dozens of children from a Catholic scout group called out "Pope Francis, Pope Francis", as they held yellow and white balloons.
"I know that many children are praying for me; some of them came here today to 'Gemelli' as a sign of closeness," Francis wrote in his message.
"Thank you, dearest children! The pope loves you and is always waiting to meet you."
