Thousands of people were gathering early on Friday in Douala, Cameroon's largest city and economic hub, for a Mass with Pope Leo that will likely be the biggest event of the pontiff's four-nation Africa tour.

The Vatican is expecting about 600 000 to fill streets around Japoma Stadium to be part of the celebration and hear an address from the pope, who has become outspoken on war and inequality and drawn the ire of US President Donald Trump.

Amid a heavy security presence, Cameroonians began filing into the stadium on Thursday, staying there overnight so they could witness Leo's homily in person.

Leo, the first US pope, on Thursday criticised leaders who spend billions on wars and, in unusually forceful remarks in Cameroon, said the world was "being ravaged by a handful of tyrants".

Leo was due to land in Douala around 9:55 am, after a one-hour helicopter flight from Yaounde, Cameroon's capital. He will spend about four hours in Douala, where he will also visit a Catholic hospital, before returning to Yaounde.

On a 10-day tour across Africa, the pontiff has also decried violations of international law by "neocolonial" world powers and said "the whims of the rich and powerful" threaten peace.

Cameroon, an oil- and cocoa-producing country, faces grave security challenges, including a simmering Anglophone conflict in which thousands of people have been killed since 2017.

Crowds greeting the pope on his visit have been enthusiastic, lining the streets along his routes and wearing colourful fabrics featuring images of his face.

Bishop Léopold Bayemi Matjei called Leo's visit "a moment of great joy" and said he hoped it meant God would bless Cameroon.

"Our country needs a lot of blessing, a powerful blessing, so that hope will come to rise again," said the bishop, who leads the Church in Obala, about an hour north of Yaounde.