Pope Leo returns to Rome on Thursday after wrapping up an ambitious four-nation Africa tour in which he forcefully decried the direction of global leadership, denouncing despotism and war, and drew the ire of US President Donald Trump.

The first US pope closed the nearly 18 000 km tour with a final Mass in a stadium in Equatorial Guinea, where tens of thousands began gathering in pouring rain before dawn for a last chance to see him.

Leo told worshippers in a homily, his 25th speech over the 10-day tour, that the Christian message means "every people is set free from the slavery of evil". He urged them to live their faith with joy.

The pope has taken on a new forceful speaking style during his time in Africa, in which he also visited Algeria, Cameroon and Angola.

Leo has warned that the whims of the world's richest threaten peace, decried violations of international law by "neocolonial" global powers, and said the world was "being ravaged by a handful of tyrants".

Trump attacked Leo as "terrible" on April 12, on the eve of Leo's Africa tour, in an apparent response to the pope's criticisms of the US-Israeli war on Iran. He lobbed several more critiques throughout the first week of the tour.

Leo told Reuters on April 13 that he would keep raising his voice, despite Trump's criticism. He later clarified to reporters that the speeches for the tour were written weeks ago, and not aimed directly at Trump.

The pope, departing shortly after midday on Thursday, is due to arrive at Rome's Fiumicino airport a little before 8 pm. He is expected to hold a news conference aboard the flight.