Bernie Sanders issued a global call to action at the Vatican on Friday to address "immoral and unsustainable" wealth inequality and poverty.

The US Democratic senator from Vermont cited Pope Francis and St John Paul II repeatedly during his speech to the Vatican conference commemorating the 25th anniversary of a landmark teaching document from John Paul on social and economic justice after the Cold War.

Sanders arrived in Rome hours after wrapping up a debate in New York, saying the opportunity to address the Vatican conference was too meaningful to pass up. The roughly 24-hour visit precedes Tuesday's crucial New York primary, which Sanders must do well in to maintain a viable challenge to Democratic frontrunner Hillary Clinton.

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Pope Francis apologised that he could not personally greet participants at the Vatican conference. No meeting with Sanders was expected.

But the trip gave Sanders a moment on the world stage, placing him alongside priests, bishops, academics and two South American presidents.

Sanders has been at a disadvantage during his campaign against Clinton, President Barack Obama's former secretary of state, on issues of foreign policy, but he was peppered with questions from academics and ecclesiastics in a manner that might have been afforded a head of state.

Sanders trails Clinton in the Democratic primaries, but the trip to the Vatican and his massive rally earlier this week with 27,000 people in New York City may have offered a glimpse of the senator's aim to become a progressive leader - win or lose.

The discussions gave him a chance to expand on his core campaign messages about the need to reform banking regulations, campaign finance rules, and higher education. Asked about inequality in public education, he said it was "beyond disgraceful" and cited challenging conditions in Detroit's school system.

He told the audience that rather than a world economy that looks out for the common good, "we have been left with an economy operated for the top one percent, who get richer and richer as the working class, the young, and the poor fall further and further behind".

Sanders also warned that youth around the world are no longer satisfied with the status quo, which includes "corrupt and broken politics and an economy of stark inequality and injustice".

Sanders' message calling for a political revolution to address wealth inequality and the influence of Wall Street on US politics has galvanised many Democrats and independents.

Reverend Matt Malone, editor of the Jesuit magazine, America, said Sanders' trip was unlikely to have much of an impact on Catholic voters, noting that conferences like the one Sanders is attending "happen all the time".

"I don't think that Bernie Sanders going to the Vatican is going to help Bernie with Catholics any more than Ted Cruz going to a matzo factory is going to help him with the Jewish vote," Malone told the AP news agency.

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Source: AP