There were ecstatic scenes in Rio de Janeiro as Pope Francis arrived on Monday at the start of a weeklong visit to Brazil for World Youth Day.

At one point the car he was travelling in from the airport was mobbed by excited fans when the driver took a wrong turn onto a busy road.

Despite the crowd surge, Vatican spokesperson the Reverend Federico Lombardi said the Pope's safety was never in doubt.

"His secretary [who was travelling in the car with him] was afraid, but the Pope was happy, with his hand out the window waving," Rev Lombardi said.

"There are no concerns for security. The concerns are that the enthusiasm is so great that it's difficult to respond to so much enthusiasm for the Pope. But there is no fear and no concern."

The journey through the city proceeded more calmly when the Pope was transferred to his open-roofed Popemobile.

He was officially welcomed to Brazil at Guanabara Palace, where he was received by President Dilma Rousseff, the Governor of Rio State Sergio Cabral Filho, and Rio Mayor Eduardo Paes.

In his speech, Pope Francis spoke warmly of the "deep sentiments of faith and friendship" that characterise the Brazilian nation and the "great heart" of the Brazilian people.

He also praised the commitment of young Catholics coming from across the globe for World Youth Day.

"Young people are the window through which the future enters the world, thus presenting us with great challenges," he said.

"Our generation will show that it can realise the promise found in each young person when we know how to give them space; how to create the material and spiritual conditions for their full development; how to give them a solid basis on which to build their lives; how to guarantee their safety and their education to be everything they can be; how to pass on to them lasting values that make life worth living; how to give them a transcendent horizon for their thirst for authentic happiness and their creativity for the good; how to give them the legacy of a world worthy of human life; and how to awaken in them their greatest potential as builders of their own destiny, sharing responsibility for the future of everyone."