Pope Francis has left for Rio de Janeiro, embarking on his first foreign trip as pontiff to attend an international youth festival in Brazil, the world's biggest Catholic country.

Francis was seen off by Italian Prime Minister Enrico Letta as he departed from Rome's Fiumicino airport on Monday morning.

In keeping with his trademark simplicity, the 76-year-old carried his own hand luggage onto the plane.

Meanwhile, pilgrims from around the world were gathering in Rio for World Youth Day, arriving by bus from neighbouring nations or landing by plane from across the ocean to greet the first pope from Latin America.

More than one million people are expected to pack the white sands of Copacabana beach where the pope will greet throngs of young people on Thursday.

Authorities are deploying 30,000 troops and police in the crime-riddled city.

Nuns checked in at hotels while other pilgrims walked on the beach, flaunting the colours of their countries.

'Eternally simple'

"We have a Jesuit pope who is eternally simple, humble, who is revolutionising the Catholic Church," said Antonio Prada, a 27-year-old Venezuelan clad in a T-shirt in his country's yellow, blue and red colours.

"His message is that we should be like Christ, that he's the example to follow," Prada said.

Speaking from the Vatican on Sunday, Pope Francis said: "Everybody who is going to Rio wants to hear Jesus. And they want to ask him: 'Jesus, what must I do with my life, what is my path?'"

The Argentine pope's message of a "poor Church for the poor", may strike a nerve in the emerging power facing a shrinking Catholic flock and anger over government waste.

Brazil has become richer in time but struggles with corruption and lagging public services that brought about one million protesters to the streets last month.

Despite the past protests, which were sometimes marred by violence, the pope is ditching his armoured "Popemobile" for an open-top jeep to have direct contact with the people.

He will start his trip with a meeting with President Dilma Rousseff, followed by a visit to one of Rio's sprawling favelas, or slums.

Source: Agencies