* Pope takes private day after tumultuous welcome
* Security concerns rise after mob blocked papal car on arrival
* Francis to fly to shrine, visit ex-addicts in hospital
Tens of thousands of young Catholics from around the world flocked to Rio's famed Copacabana beach on Tuesday to kick off a youth festival that will feature several appearances by Pope Francis.
Bikinis and beach volleyball gave way to rosaries and prayers for world peace as youths settled on the urban shoreline for a night of religious events related to World Youth Day, a biennial gathering of young Catholics.
The 76-year-old pope, the first pontiff from Latin America, took a rest day on Tuesday after a tumultuous welcome during his arrival on Monday that prompted security concerns when police failed to contain frenzied crowds that mobbed his car.
As Brazilian officials worked to avoid a repeat of such incidents for the remainder of his weeklong visit, Catholic youth, waving their national flags and singing religious songs, gathered along the beach for an evening Mass.
Musicians and dancers entertained the youths, who braved a chilly drizzle. The beach, lined with a series of large monitors, was the site of an evening Mass by local priests and Vatican officials. Later in the week it will serve as an altar for two big appearances by Francis.
A metro malfunction in the evening left some stations out of service for two hours, leaving some people stranded on their way to the inaugural Mass.
Francis, meanwhile, rested at a Church residence and recovered from jet lag ahead of a rigorous five days during which he will preside at about a dozen events before returning to Rome on Sunday night.
The security scare on Monday occurred when his driver took a wrong turn in central Rio. The street had no barriers, which allowed a crowd of well-wishers to swarm the car and reach through the open window to touch the pontiff. Bodyguards moved in to push back the crowd. At one point, the crowd was so heavy that the car was forced to a halt.
On Wednesday, the pope flies by helicopter to Aparecida, about 260 km (162 miles) away, to visit the shrine of the Madonna of Aparecida, whom Catholics venerate as the patroness of Brazil.
In the afternoon, he returns to Rio to visit a hospital run by the Franciscan religious order and address young people taking part in a drug rehabilitation program.
Benedict XVI, now "pope emeritus," was scheduled to make the trip to Brazil for the youth festival before he resigned on Feb. 28, becoming the first pope to step down in 600 years.
The visit to the Aparecida shrine, the hospital and a slum once controlled by drug lords on Thursday were added to the program at Francis' request after he was elected on March 13.