Pope Francis is due in Kenya at the start of a three-nation African tour – his first to the continent as pontiff.
Thousands are expected to line the streets of the capital, Nairobi, to welcome him for his three-day stay.
An atheist group says it will challenge in court a government decision to declare Thursday a holiday in honour of the pontiff, local media reports.
A leading Muslim cleric in Kenya welcomed the visit, saying it gave hope to the “downtrodden in the slums”.
Pope Francis is also due to visit Uganda and Central African Republic, which has been hit by Christian-Muslim conflict.
The BBC’s Joseph Odhiambo in Nairobi says about 30% of Kenyans – including President Uhuru Kenyatta – are baptised Catholics, and there is huge excitement around the visit.