VATICAN CITY • Pope Francis has landed in Kenya on a trip aimed at pushing the key themes of his papacy but which is also fraught with security concerns.

The 78-year-old will use his 11th overseas trip since taking office in 2013 to deliver a total of 19 speeches on peace, social justice, environmental protection and interfaith dialogue in Kenya, Uganda and the Central African Republic (CAR).

On the streets of Kenyan capital Nairobi, huge billboards have been erected to welcome the pontiff. Vast crowds were expected to turn out to see his motorcade yesterday.

Predictions that the final leg of the trip, to the conflict-wrecked CAR, would be postponed have so far proved off the mark, despite warnings that his security cannot be guaranteed. Vatican officials say a change of programme will happen only if there is a precise threat that could endanger the thousands of believers expected to come and see him, many of whom will be travelling from neighbouring countries.

The Pope is due to be welcomed in Kenya by President Uhuru Kenyatta. In a speech at the presidential residence State House, he is expected to address questions of corruption and the gulf between rich and poor, both issues of concern for Kenya.

Pope Francis is scheduled to use an open-topped popemobile regularly during a trip that will take in a Kenyan shanty town and a refugee camp and mosque in the CAR at a time when security concerns have been raised to red alert levels following militant attacks in Paris, Beirut, Egypt and Mali.

Aides say he is determined the sombre backdrop will not affect his plans, particularly for the CAR part of the trip, where he is due to open a "Holy Door" in capital Bangui's cathedral. The opening of the door will powerfully reflect the Pope's concern for those on the fringes of the Catholic community and his desire to create a "poor Church for the poor", said Vatican experts.

But it may not happen. CAR's acting president Catherine Samba-Panza may cut the visit to a few hours in Bangui airport, the one area the French military say they can be fairly sure of protecting the leader of the world's 1.2 billion Catholics.

The Pope is said to be stubbornly resisting any curtailment of his schedule but ultimately it is his security advisers who will decide.

He is the third pope to visit Africa, a continent which now produces one in six of the world's Catholics.

There will also be interest in his comments when he visits the Nairobi headquarters of the UN's Environment Programme and Human Settlements Programme.

He is a fierce environmentalist and is unlikely to temper his words when discussing global warming, putting heat on world leaders ahead of climate change talks in Paris, which will begin on Monday.

AGENCE FRANCE-PRESSE