Over half a million young Catholics packed Rio's Copacabana beach in Brazil on Tuesday for the start of a week of religious events, that include gatherings with Pope Francis.

560,000 pilgrims waved flags, sang and prayed as the archbishop of Rio led a mass to kick off World Youth Day.

The Pope was taking the day off one day after a rapturous welcome on Monday, but he tweeted greetings to his flock urging them to 'Go and make disciples!"

Hours before the Mass started, the city's metro system broke down due to a power failure, forcing people to cram into buses, and taxis or walk to the beach.

Some 1.5 million people from 170 countries are expected to attend World Youth Day, which actually lasts a week, and wraps up on Sunday.