(AFP) - Pope Francis voiced hope for a "fruitful and joyous" old age on Saturday, as he kicked off his 80th birthday by sharing breakfast with a group of homeless people.

"Old age sounds ugly, and causes fear. But old age yearns for wisdom," he told dozens of cardinals at the mass in a papal chapel in Rome.

"Old age is quiet and religious, but also fruitful. Pray that mine will be that way, quiet and religious, fruitful and also joyous."

The pope began his birthday before mass by inviting eight homeless people to share his breakfast, including pastries from his native Argentina.

He also had pastries sent to other soup kitchens across Rome, and sent small gifts to residents in the city's emergency shelters, the Vatican said.

More than 50,000 messages of congratulations streamed in from around the world, including at email addresses in eight languages specially set up by the Vatican.

President Barack Obama said Pope Francis had "in both word and deed... inspired people around the world with his message of compassion, hope, and peace".

"Italy is grateful to you for your constant closeness," said Italian President Sergio Mattarella, while the country's new prime minister, Paolo Gentiloni, expressed "gratitude to the Holy Father for the inspirational force of his message and example".

Among the gifts streaming in were drawings by children, Jesuit theologian Antonio Spadaro, who is close to the pontiff, tweeted.

Spadaro released photographs of drawings showing a smiling pope with the world on his head, or surrounded by children and party balloons.

- Hardworking -

But like almost every other day of his papacy, Saturday was mainly spent as a working day for the holiday-phobic Francis.

Apart from the morning mass with the cardinals, he held meetings with Maltese President Marie-Louise Coleiro, several prelates and members of the Nomadelfia, a secular community that tries to live as the early Christians did.

In recent years, the pope has marked his birthday by distributing hundreds of sleeping bags to homeless people in Rome and by sending food to a migrant shelter.

Born into a family of Italian heritage in 1936, Jorge Bergoglio became the 266th pope when he was elected on March 13, 2013.

Saturday also marks the fourth birthday the former bishop of Buenos Aires has celebrated in the modest St Martha's boarding house which he has made his home inside the Vatican's walls.

He has shown no sign of slowing down in his drive to reshape the Catholic church to reflect his own vision of compassion and humility.

The first pope from the Americas continues to set a relentless pace as he reaches a milestone at which cardinals are ushered into semi-retirement.

There are signs of fatigue, natural for a man of his age who lost part of a lung in his youth, and the occasional grimace bears witness to the sciatic pain that is a near constant companion.

He has not ruled out following the historic example of his predecessor Benedict XVI, who retired, exhausted, in 2013, although there is no suggestion that could happen soon.

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