Around 400,000 Roman Catholics have joined a ceremony on Copacabana Beach in the Brazilian city of Rio de Janeiro to mark the opening of World Youth Day.
The highlight of the festival will be a visit on Thursday by Pope Francis.
The Pope arrived in Brazil on Monday, making his first trip abroad since becoming head of the Roman Catholic Church.
His visit is taking place under tight security, after weeks of protests against the government and corruption.
Tuesday’s evening Mass at Copacabana Beach was led by Rio’s Archbishop Orani Joao Tempesta.
He welcomed pilgrims to the five-day festival, which is expected to draw about 1.5 million people from around the world.
The Pope, who is from neighbouring Argentina, had no public events scheduled for Tuesday and spent time at a private residence.
He is expected to greet large crowds at Copacabana on Thursday.
There was transport chaos shortly before the event when the Rio metro system was brought to a standstill by a power cut.
The city’s two metro lines went down for two hours after a power cable broke, a Rio Metro statement said.
Crowds of people hoping to attend the Mass were left stranded, while others crammed onto buses and taxis heading to the beach.
Police stepped in to block a group of angry passengers from rushing into one of the main metro stations, local radio reported.
Correspondents say Brazil is reviewing security around the 76-year-old pontiff after he was mobbed following his arrival in Brazil on Monday.
Many were able to stop the Pope’s motorcade as it travelled through Rio and reach their hands inside his car’s open window.
“The Pope’s secretary told me he was terrified, but the Pope kept smiling,” Vatican spokesman Federico Lombardi told reporters.
The Pope was officially welcomed at the state governor’s palace by Brazil’s President Dilma Rousseff on Monday.
However, police outside later fired tear gas to disperse people who were protesting against the government, but also against the cost of the papal visit.
On Wednesday the Pope will visit the Shrine of Our Lady of Aparecida in Sao Paulo state.
A homemade explosive device was discovered there on Sunday, but the authorities said it was “of low power” and nowhere near the area where the Pope and pilgrims will visit.
Mr Lombardi later insisted that the discovery was no cause for concern.
At his request, the Pope will also visit a hospital in Rio on Wednesday and address young people taking part in a drug rehabilitation program.