Pope Francis was greeted by enthusiastic crowds in Rio de Janeiro as he returned to his native continent for the first time as pontiff, but was involved in a security scare as his car took a wrong turn on the way from the airport.
Later petrol bombs were thrown and protesters accused riot police of an unprovoked attack in clashes outside the state governor's palace.
The pope landed in Rio at 4pm local time and toured streets in the city centre. During his first minutes in Brazil, believers swarmed around the closed Fiat several times when it was forced to stop by heavy traffic on the drive from the airport to an official ceremony in Rio's centre.
A few security guards struggled to push the crowd back. Church and city officials said the pope's driver turned into the wrong part of a boulevard and missed lanes that had been cleared.
Other parts of the pope's route to the city centre were not lined with fencing, giving the throngs more chances to get close, with uniformed police nowhere in sight to act as crowd control.
Francis looked calm during the frenzy. He rolled down the window on the car where he was sitting, waving to the crowd and touching those who reached inside. At one point, a woman handed the pontiff a dark-haired baby, whom he kissed before handing it back.
"His secretary was afraid," papal spokesman the Rev Federico Lombardi said. "But the pope was happy."
The Vatican insisted they had no concern for the pope's safety as his vehicles eased through the masses, but Lombardi acknowledged that there might have been some "errors" that need correcting. "This is something new, maybe also a lesson for the coming days," Lombardi said.
After finally making it past crowds and blocked traffic, Francis switched to an open-air popemobile as he toured around the main streets in downtown Rio through mobs of people who screamed wildly as he waved and smiled. Many in the crowd looked stunned, with some standing still and others sobbing loudly.
"This is the youth of the pope!" chanted young pilgrims outside Rio's Metropolitan Cathedral. Others sang "I'm Brazilian, with much pride, with much love," while helicopters hovered overhead and troops and armed police lined the streets.
"He is a person of faith and he has shown his greatness in a very short time," said Diego Moreno, who had travelled with two friends from Mendoza in Argentina. "We are very proud of him."
Up to 2.5m pilgrims are expected to crowd events such as another parade down Copacabana beach front and a two-day prayer vigil and mass in the so-called "Field of Faith" at Pedra de Guaratiba.
In a speech beside President Dilma Rousseff at the state government seat, Laranjeiras Palace, Pope Francis charmed his Brazilian hosts with his humility.
"I learnt that to have access to the Brazilian people, it is necessary to enter through the door of their immense heart. Permit me at this time to knock delicately at this door," he told his audience.
"I have neither silver nor gold, but I bring with me the most precious thing given to me: Jesus Christ."
Violence broke out around 7.30pm outside the Guanabara Palace just after the pope had left a reception with President Dilma Rousseff.
A number of demonstrations, including a gay rights march, had joined together into a crowd of some 2,000. The march come to a halt in front of massed ranks of riot police and an armoured vehicle outside the palace.
About an hour after the pontiff concluded his short speech, police began cracking down on the protests, firing rubber bullets, tear gas and water cannon in an effort to disperse the crowd. Both sides accused the other of provoking violence.
Police said they seized 20 home-made petrol bombs from one arrested demonstrator and released a video showing what they said were protesters throwing a petrol bomb at the police line. News agency photos published on Globo's G1 news site showed a petrol bomb hitting a policeman.
Demonstrators said the police reaction was overwhelmingly aggressive. "Police attacked. People were overcome with despair. There was no way out. They wanted people to suffer terror," said protestor Luiza Dreyer.
Daniela da Sousa, who described herself as Catholic and said she was taking part in World Youth Day, said she saw the pope and other dignitaries leaving the palace and that police began attacking protesters as soon as they had gone.
"The authorities left, and the bombs started. They attacked people," da Sousa said. "I am Catholic. I saw this cowardice. I wanted to rip up this T-shirt," she added, pointing to the World Youth Day T-shirt she was wearing. "It is disgusting."
As the violence abated, about 500 protesters marched to a police station in nearby Catete to demand the release of reporters from the Midia Ninja internet television station who were among 10 who had been arrested.
With TV Globo increasingly under attack by protestors, Midia Ninja has rapidly become a trusted source of information for many involved in the protest movement and has broadcast live from protests all over Brazil.
There was a tense stand-off as a line of riot police stood in front of the police station while the crowd shouted for the release of the reporters. When one of them, Felipe Peçanha, was released he was mobbed by the crowd who chanted: "Ninja! Ninja!"
Peçanha told the Guardian he had been arrested by plain clothes police as he was broadcasting live in the midst of the confusion and put into a police car.
"They put me with force in the police car," he said. "They asked me to stop recording, they said recording was forbidden there, and that I had to stop the transmission. I said I would not disconnect and they took the cellphones from my hand by force."
Rio newspaper O Globo reported that two police, a Japanese photographer, and Globo photographer Marcelo Carnival were injured during the conflict. Four people were treated in hospital, two for rubber bullet wounds.
