VATICAN CITY — Walking up to crowds, shaking hands with surprised bystanders in the street, mixing his formal speeches with off-the-cuff remarks, Pope Francis stamped his own style on the papacy Sunday.

His humor and down-to-earth manner captivated those filling St. Peter’s Square in Rome to overflowing, and he worked the crowd in a way that had to give his security staff palpitations. Rome Mayor Gianni Alemanno, in the square himself, estimated the crowd’s size at 300,000.

‘’Brothers and sisters, ‘Buon giorno,”’ Francis said in Italian in his first welcome from the window of the papal residence, setting an informal tone that has become the defining spirit of his young papacy.

Earlier Sunday, he made an impromptu appearance before the public from a side gate of the Vatican that startled passers-by and prompted cheers as he shook hands and kissed babies. Francis had just finished celebrating Mass and delivering a six-minute homily — brief by church standards — in the Vatican’s tiny parish church, St. Anna, when he walked outside to greet parishioners one by one, just as an ordinary pastor does after weekly services.

Francis started speaking at the window even before the stroke of noon — the appointed time for the weekly papal address.  The windows of the papal study in the Apostolic Palace were opened for the first time since Francis’ predecessor, Benedict XVI, gave his last Sunday blessing on Feb. 24. Four days later, Benedict went into retirement, the first pontiff to do so in nearly 600 years.

Francis, the first pope from Latin America, was elected Wednesday and has been staying in a hotel on the Vatican’s premises until the papal apartment is ready.

“The pope is down-to-earth. He is a people person and it is amazing,” said Emanuel Anatsui from Britain. “He is going to do wonderfully for the church.”

After Mass, Francis again put his security detail to the test as he waded into an intersection just outside St. Anna’s Gate. Francis stepped up to the crowd, grasping outstretched hands. The atmosphere was so casual that several people even gripped Francis on the shoulder.

“Francesco! Francesco!” children shouted his name in Italian. As he patted one little boy on the head, he asked “Are you a good boy?” and the child nodded.

“Are you sure?” the pope quipped.

At one point he glanced at his watch and turned to an aide — as if to ask ‘’How much time do I have?”

The pope then ducked back inside the Vatican’s boundaries to dash upstairs for the address to St. Peter’s Square.

Often abandoning the prepared text in his hand, Francis told the crowd that he wanted to talk about mercy, saying he was inspired by a book about forgiveness that he was reading. Citing the author, an elderly German cardinal, and praising him as a ‘’top-notch” theologian, Francis quipped: ‘’Don’t think I’m making publicity for my cardinals’ books!” drawing a roar of laughter from the crowd.

Francis said mercy can ‘’change the world” and make it “less cold and more just.”

He spoke only in Italian — ending with “Buon pranzo” (Have a good lunch) — a wish that triggered nods of approval from the crowd in Rome, where a leisurely Sunday family lunch is a cherished tradition.

But Francis did tweet in English and other languages, saying: “Dear friends, I thank you from my heart and I ask you to continue to pray for me.”’

Past pontiffs have used the Sunday window greetings to offer brief reflections and wishes in several languages.

Vatican spokesman the Rev. Federico Lombardi said Francis would likely stick with Italian, a language he’s comfortable with for spontaneous remarks. Lombardi left open the possibility the 76-year-old pope would use other languages in future public appearances.

During his window speech, Francis also talked about of his family’s roots in Italy’s northwestern Piedmont region. He told the crowd that by naming himself as pope after St. Francis of Assisi, an Italian patron saint, he was ‘’strengthening my spiritual tie with this land, where, as you know, my family has its origins.”

The crowd was cheering wildly when Francis appeared at the window, but fell into rapt silence when he began to speak. Some people’s eyes welled up. Many people waved the blue-and-white flag of Argentina, the pope’s homeland. Some people held their children aloft or on their shoulders to get a better look.

“We are so proud. He is Argentine, but also belongs to the rest of the world,” said Ivana Cabello, 23, of Argentina.

Angela Carreon, a 41-year-old Rome resident originally from the Philippines, ventured that Francis “looks like John Paul II. “

“I hope he is like him,” she said. “He has a heart.”

The globe-trotting Polish-born John Paul II, who died in 2005, loved to charm the crowds.

Several hundred extra traffic police were deployed Sunday to control crowds and vehicles for Francis’ first window speech as well as the annual Rome marathon. Bus routes were rerouted and many streets were closed off to channel the curious and the faithful up the main boulevard from the Tiber river to St. Peter’s Square.

Giant video screens were set up so the huge crowd could get a close look at Francis, and dozens of medical teams were on hand for any emergencies. In the last hour before noon, a large backup formed of people trying to squeeze through three openings in the fence ringing the front of the square. But by the time Francis appeared, all had calmly found a viewing spot.

Among Francis’ first formal meetings is an appointment Monday with Argentine President Cristina Fernandez. That will provide an opportunity to see if the new pope’s easygoing manner still holds — the two have been on opposite sides for many years. As Buenos Aires archbishop, Francis had lobbied hard against the government’s move to legalize gay marriage and make contraceptives available for free.

On Tuesday, Fernandez will join other world leaders and senior international envoys, including U.S. Vice President Joe Biden and the president of Jesuit-run Georgetown University, for Francis’ formal installation as pope.