Pope Francis took off from Rome's Fiumicino airport on Wednesday aboard an Alitalia A330-200 airbus bound for Kenya, on the first leg of his inaugural six-day visit to three African countries.
"Mungu abariki Kenya! God bless Kenya!" the Francis tweeted on his @Pontifex account.
He was due to arrive at Nairobi's Jomo Kenyatta airport at 5 pm local time. Thousands were expected to line the streets of the capital, Nairobi, to welcome him for his three-day stay.
The government has declared Thursday a public holiday in honour of the pontiff's visit and up to 10,000 police officers may be deployed during the visit, the government said.
"Italy and the entire international community is watching your first trip to Africa, whose potential for growth and development is stunted by war, political instability, poverty and shocking social inequality, Italy's president Sergio Mattarella said in a message to Francis on Wednesday.
Francis' presence would cheer Christian communities and send "an important message of peace, fraternity and dialogue" in the countries he visited, Mattarella added.
A leading Muslim cleric in Kenya welcomed the visit, saying it gave hope to the "downtrodden in the slums".
Abdalla Kwamana, the vice-chairman of the Supreme Council of Kenya Muslims described the visit as highly significant, and welcomed the Pope's decision to include a shantytown in his itinerary.
Around 30 percent of Kenyans - including president Uhuru Kenyatta - are baptised Catholics, and there is huge excitement around the visit, observers say.
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 kick off with a meeting with Kenyatta.
The pope's African trip will also feature visits to Uganda and the Central African Republic, an active war zone which has been hit by Christian-Muslim conflict and there had been speculation the CAR leg would be cancelled.
The Pope's current CAR schedule includes a visit to a mosque in a volatile part of the capital, Bangui, but his intinerary will be under constant review, said his spokesman, Federico Lombardi.
Francis plans to ride in an open popemobile in Bangui, Lombardi said, while denying reports he would wear a bulletproof vest.
Security for the visit has been tightened since the recent terror attacks in Paris and in Mali's capital Bamako, the Vatican acknowledged.
Vatican security chief Domenico Giani was due to travel to CAR to assess the situation before the pope’s arrival.
One in six of the world's Catholics live in Africa.
