Pope Francis on Tuesday expressed his thanks for "the magnificent welcome" offered to him by this Brazilian metropolis, where he arrived hours earlier to participate in the 28th World Youth Day event.

"Thank you! Thank you! Thank you to all of you and all the authorities for the magnificent welcome (to Rio de Janeiro)," the pontiff wrote on Twitter after having spent his first night in the city.

"Today we begin a stupendous week in Rio. May it be an occasion (on which) to deepen our friendship with Jesus Christ," said the Argentine pope.

World Youth Day activities were to be inaugurated officially on Tuesday with a Mass on Copacabana beach, though without the presence of the pope.

Francis arrived in Rio on Monday after a 12-hour flight from Rome and later he rode through the city's main streets amid fervent crowds who thronged the avenues to get a glimpse of him.

A mistake in the route at one point got the pontiff's automobile trapped in a traffic jam, which youths from dozens of countries who have gathered in Rio for Youth Day took advantage of to approach Francis and even touch him, causing some desperate moments for his security team. EFE