Vatican City, Holy See — Catholic cardinals in Rome marked the start of the Lent religious season on Wednesday without Pope Francis, as the 88-year-old spent his 20th day in hospital seriously ill with pneumonia.

The Argentine pontiff, head of the worldwide Catholic Church, usually leads the Ash Wednesday celebrations marking the start of 40 days of prayer and sacrifice before Easter, the holiest period in the Christian calendar.

Instead Italian Cardinal Angelo De Donatis read out the pope's homily at the mass at Rome's Santa Sabina basilica, which was preceded by a procession joined by around 20 red-clad cardinals.

"We feel deeply united with him at this moment and we thank him for the offering of his prayers and his sufferings for the good of the entire Church, and the whole world," De Donatis said of the pontiff at the start.

Francis, leader of the world's almost 1.4 billion Catholics, was admitted to Rome's Gemelli hospital on February 14 with bronchitis, which then developed into pneumonia in both lungs.

He has since suffered a worrying series of crises, including "two episodes of acute respiratory failure" on Monday, which have sparked widespread alarm.

The Vatican said Tuesday evening Francis was in a "stable" condition with no fever, but once again said his prognosis "remains reserved", meaning doctors will not say how they expect his condition to evolve. 

In Wednesday morning's update, it said the pope had "rested well" overnight.

A Vatican source added later in the day that he had slept with an oxygen mask covering his nose and mouth for the second night in a row, before switching to "high-flow" oxygen via a nasal cannula in the morning.

Francis has not been seen since his hospitalisation, with the pope notably missing three successive Sunday Angelus prayers -- a first in his 12-year papacy.

He had previously missed Ash Wednesday celebrations in 2022, that time due to acute knee pain, one of a series of health woes that have afflicted the pontiff in recent years.

'We really need him'

Francis's health has regularly led to speculation, particularly among his critics, as to whether he could resign like his predecessor, Benedict XVI, who stepped down in 2013.

Pilgrims and well-wishers have been gathering in front of the hospital to light candles and pray for Francis, a liberal reformer who has sought to forge a more compassionate Church, notably by reaching out to the most disadvantaged in society.

Domenica Patania, a 63-year-old Italian, told AFP she was praying for him to get better "because in this particular period of time we really need him, his support and his closeness".

"Above all, we want him to be well for many, many years to come."

Francis, who had part of a lung removed as a young man, had been breathless and struggled to read his texts in the days leading up to his hospitalisation.

On 22 February, he suffered a "prolonged asthmatic respiratory crisis" and on 28 February had "an isolated crisis of bronchospasm" -- a tightening of the muscles that line the airways in the lungs.

On Monday, Francis "experienced two episodes of acute respiratory failure, caused by a significant accumulation of endobronchial mucus and consequent bronchospasm", according to the Vatican.

Acute respiratory failure, which can be life-threatening, occurs when the lungs cannot pass enough oxygen into the blood or when carbon dioxide builds up in the body.

Francis is in a special papal suite at the hospital with its own chapel. His medical team has not commented on the length of his stay, nor how long his recovery could take.