Pope Francis has arrived in Kenya for a three-nation African tour – his first to the continent as pontiff.
President Uhuru Kenyatta and ululating crowds welcomed him at the airport in the capital, Nairobi.
An atheist group says it will challenge in court a government decision to declare Thursday a holiday in honour of the pontiff.
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, BBC reported.
“I go with joy to meet Kenyans, Ugandans and our brothers in Central Africa,” he told journalists on his plane, the Associated Press news agency reports.
He played down fears for his safety by joking: “I’m more worried about the mosquitoes.”
Kenya’s government has said that up to 10,000 police officers may be deployed during the visit.
Militant Islamists have carried out a spate of attacks in Kenya – including the 2013 siege at Nairobi’s Westgate shopping centre, which left at least 67 dead, and the killing of about 150 people during an assault on the Garissa National University College in April this year.
The BBC’s Joseph Odhiambo in Nairobi says about 30% of Kenyans – including President Kenyatta – are baptised Catholics, and there is huge excitement around the visit.
Crowds have lined the streets of Nairobi to catch a glimpse of the pope as he was driven in a grey Honda saloon to Mr Kenyatta’s office for his official engagement.
One of Kenya’s main newspapers, The Standard, welcomed him with headlines in Latin, Grata Franciscus Pontifex, and the regional Swahili language, Karibu Papa Francis.
Crowds have lined the streets of Nairobi to catch a glimpse of the pope as he drove in a black car to Mr Kenyatta’s office for his official engagement.
The Pope is expected to tackle corruption, poverty and religious conflict during his visit, which will start with a meeting with President Kenyatta.
‘Consoling poor’
Abdalla 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.
One in six of the world’s Catholics are in Africa.