Vast crowds of Ugandans greeted Pope Francis on Saturday as he honoured Christians martyred for the faith on the second leg of a landmark trip to Africa, which he dubbed "the continent of hope".

The 78-year-old pontiff received a rapturous welcome as he arrived at a shrine to the martyrs at Namugongo, just outside the capital Kampala, where some 45 Christians were executed in 1886 for refusing to recant their faith.

Wild cheers and singing broke out as Francis arrived at the open air shrine. Over 100,000 people had waited from before dawn to attend the mass, a key highlight of his visit to Uganda.

Later, he will meet with young Ugandans at a huge ceremonial ground in the capital's Kololo district.

Catholic faithful from neighbouring war-torn South Sudan also came for the mass, travelling for 12 hours by bus to catch a glimpse of the Argentine pope, who has made humility and help for the poor a hallmark of his tenure.

Despite the hot and muggy weather, Jonathon Ssali said he had managed to sell about 55 souvenir scarves during the morning, each one emblazoned with the pope's image alongside the red, black and yellow of the Ugandan flag.

"This is helping me make a little money," said the 21-year-old, who came up with the idea of selling the souvenirs, each costing Ush10,000 ($3/2.80 euros), in June.

"I'm excited, it's my first time to see a pope. When Pope John Paul II visited in 1993, I was just about to be born," said Ssali, who is Protestant.

"I want to hear his powerful words of unity and compassion," said 37-year-old Anthony Beda from South Sudan, wearing a pope badge and waving a flag with the pontiff's face on it.

"If there's one country he should visit, it's South Sudan," Beda told AFP, saying it could help stop the civil war there. "There's a lot of corruption and conflict there. I would love him to go... It would be a blessing."