Democratic front-runner Hillary Clinton.

He told the audience of priests, bishops, academics and two South American presidents that rather than a world economy that looks out for the common good, "we have been left with an economy operated for the top 1 percent, who get richer and richer as the working class, the young and the poor fall further and further behind."

During a discussion later in the afternoon, Sanders was peppered with questions from academics and ecclesiastics at the conference, giving the presidential hopeful a chance to expand on his core campaign messages about the need to reform banking regulations, campaign finance rules and higher education.

"We don't choose to politicize the pope," Sanders told attendees, "but his spirit and courage and the fact, if I may say so here,his anti-imperialist, socialist rhetoric.

As he walked through Vatican City's Perugino gate, Sanders was greeted about two dozen supporters, some of whom carried signs bearing Sanders' name.

The senator told reporters that he was honored to address the conference and admired Francis' message on the economy and the environment.

"I know that it's taking me away from the campaign trail for a day but when I received this information it was so moving to me that it was something that I could just simply not refuse to attend," he said.

Pope Francis apologized that he couldn't personally greet participants at the Vatican conference. No meeting with Sanders was expected.

Sanders was accompanied on the trip by his wife, Jane Sanders, and 10 family members, including four grandchildren.

The conference was organized by a Vatican