Vatican City - Pope Leo XIV called for an end to war in the world as he greeted crowds from the balcony of St. Peter's Basilica on Sunday, in his second official public appearance since becoming the leader of the Catholic Church.

"Dear brothers and sisters, happy Sunday!" the first pontiff from the US proclaimed, addressing tens of thousands in St Peter's Square gathered underneath him who cheered, applauded wildly and enthusiastically waved flags.

The new Chicago-born pope did not shy from world events in his public address following a prayer to the Virgin Mary, as he cited the recent 80th anniversary of the end of World War II, a conflict which killed 60 million people.

"In today's dramatic scenario of a third world war in tatters, as repeatedly stated by Pope Francis, I also address the powerful of the world, repeating the ever-timely appeal: no more war!" Leo urged from the balcony.

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As had his predecessor Francis, he appealed for a "genuine, just and lasting peace" in Ukraine and said he was "deeply saddened" by events in Gaza, calling for a ceasefire as Israel continues its mass killing.

"Humanitarian aid must be provided to the exhausted civilian population and all the hostages must be freed," he said.

Addressing cardinals on Saturday, the 69-year-old Leo called himself a "humble servant of God... and nothing more than this," and an "unworthy successor" to St Peter, according to a transcript released by the Vatican.

Pilgrims and worshipers packed St Peter's Square for Leo's appearance Sunday, bearing flags and Catholic symbols from across the globe. Priests and nuns mingled with the crowds, with many groups of young people, some who had played guitars or sang songs while waiting.

Before his appearance Sunday, Leo celebrated mass at the altar near the tomb of St Peter, deep beneath the basilica within the Vatican Grottoes, the Vatican said.

In the first clues as to the direction of his pontificate, Leo said Saturday he would be driven by the legacy of his predecessor, Pope Francis – who died on April 21 aged 88 – "with his example of complete dedication to service and to sober simplicity of life."

Leo made an unannounced visit to pray before Francis's simple marble tomb inside Santa Maria Maggiore basilica in the early evening on Saturday. The church was a favorite of Francis and worshipers have continued to line up to pay their respects there.

Cardinals describe Leo as in the mold of Francis, with a commitment to the poor and disadvantaged, and a focus on Catholics in far-flung areas away from Rome.

As an Augustinian and former missionary, he also believes the Church should be committed to "the missionary conversion of the entire Christian community," as he told cardinals.

The new pope's personal style is seen to be less direct than the sometimes impulsive Francis, a progressive who shook up the Church and often ruffled feathers within the Roman Curia, or government of the Holy See, during his 12-year papacy.