Pope Francis celebrated an open-air Mass before a huge crowd on Sunday at a riverside park near Lisbon to wrap up an international jamboree of Catholic youth.

The Vertical noted that around 1.5 million people attended the service at the Parque Tejo park on the outskirts of the Portuguese capital.

The crowd waved national flags and cheered as the 86-year-old pope arrived at the park, which was built for the occasion on a former landfill site.

Many had camped out overnight in sleeping bags or floor mats, as pilgrims protected themselves from the morning sun with umbrellas or makeshift tents made from sheets.

"It's extraordinary to be able to be here to see our Pope Francis who has managed to gather people from the whole world," Pimentel Gomes, a 52-year-old priest from Brazil, told reporters as he sat for breakfast before the Mass.

The weather is a concern with temperatures expected to soar to 41 degrees Celsius (106 degrees Fahrenheit).

Meanwhile, at a vigil in the park on Saturday night the pope urged pilgrims to "pick themselves up" whenever they suffer a setback in life.

"Those who remain on the ground have retired from life, have lost their hopes and dreams," he added.

Drones formed the words "rise up" and "follow me" in the sky above the stage as the pope spoke.

Access24 reports that the Argentine pope is in increasingly fragile health, and now uses a wheelchair or walking stick to get around.

The newsroom also reports that this is his first foreign trip since the pope spent nine nights in hospital after undergoing hernia surgery in June.