(AFP) - Pope Francis thanked those who prayed for him during his illness Sunday, as he kicked off the holiest week in the Christian calendar just a day after leaving hospital following a bout of bronchitis.

"I thank you for your participation and also for your prayers, which intensified during these past days. Thank you!", he told crowds gathered for Palm Sunday in St Peter's Square.

The 86-year-old's admittance to hospital in Rome on Wednesday with breathing difficulties sparked concerns he may not be well enough to attend an important series of Easter rites.

But Francis had promised to be present, and he waved briefly to some 60,000 people as he rode in his popemobile through the square, which was adorned with more than 35,000 plants and flowers.

He looked serious as he followed the procession of religious figures, including red-robed cardinals, carrying large palm leaves and olive branches.

He blessed the palms, then walked a short way with the aid of a walking stick to the altar, where he sat while the ceremony was conducted by a cardinal.

The Vatican said this was an arrangement adopted prior to the pope's latest illness, as the Argentine is no longer able to stand for long periods.

Francis, who had appeared pale at times and spoke softly, stood to say the Angelus at midday, and the crowds applauded as he thanked them for their prayers.

He then did a final round in the popemobile, waving and smiling.

"He's a pope who dedicates himself to others... let's hope he lasts a long time!" pilgrim Giovanni, who gave no last name, said ahead of the mass.

Palm Sunday marks the arrival of Jesus in Jerusalem before his crucifixion. Easter Sunday, which this year is on April 9, celebrates his resurrection from the dead.

- Increasing health issues -

As a smiling Francis had left Rome's Gemelli hospital Saturday after a three-night stay, he quipped to well-wishers who asked how he was "I am still alive!"

The head of the world's 1.3 billion Catholics has suffered from increasing health issues over the past few years, including knee problems which have forced him to use a wheelchair and a walking stick.

Francis felt unwell Wednesday following a general audience in St Peter's Square, but his condition improved after he was given antibiotics.

The hospitalisation was his second since 2021, when he underwent colon surgery, also at Gemelli.

His increasing health issues over the past year have sparked widespread concern, including speculation that he might choose to retire rather than stay in the job for life.

Francis marked 10 years as the head of the worldwide Catholic Church last month.

He has pushed through major governance reforms and sought to forge a more open, compassionate Church, although he has faced internal opposition, particularly from conservatives.

He has repeatedly said he would consider stepping down were his health to fail him -- but said earlier this year that, for now, he has no plans to quit.

Francis' previous stay at Gemelli in July 2021 lasted 10 days. He was admitted after suffering from a type of diverticulitis, an inflammation of pockets that develop in the lining of the intestine, that required surgery.

In an interview in January, the pope said the diverticulitis had returned.

© Agence France-Presse