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.
The Pope - who is likely to travel in an open-topped vehicle - is expected to tackle corruption, poverty and religious conflict during his visit, which will start with a meeting with President Kenyatta.
Abdallam Kwamana, the vice-chairman of the Supreme Council of Kenya Muslims, said he would attend an inter-faith meeting the pontiff is hosting on Thursday.
He described the visit as highly significant, and welcomed the Pope's decision to include a shantytown in his itinerary.
"It is often said that Kenya is owned by the rich and powerful. The people in the slums are never recognised," Mr Kwamana told the BBC's Newsday programme.
"When he goes to see them and console them, they'll feel they are people of substance," he added.
Pope Francis is due to hold a mass on Thursday at the University of Nairobi sports ground, where a crowd of more than one million is expected, Kenya's private Daily Nation newspaper reports.
The tiny Atheists in Kenya group said the decision to declare Thursday a public holiday and a day of prayer was unconstitutional.
"The constitution clearly states that there shall be no state religion. We cannot have the government acting religiously," its leader Harrison Mumia said, Nation FM radio station reports.