Harrowing accounts of girls forced into prostitution and modern slavery opened a two-day conference at the Vatican Tuesday, at which mayors from around the world testified to their efforts to fight forced labour and global warming.

"It's not possible that it still exists, that we remain blind" to the issue of modern slavery, Ana Laura Perez Jaimes, who spent five years chained up and forced to work 20 hours a day in Mexico, told the meeting organised by Pope Francis.

Fellow Mexican Karla Jacinto, physically and sexually abused by her family, described how she was trapped into a cycle of exploitation between the ages of 12 and 17, when she was forced to have sex with more than 42,000 clients.

Around 60 mayors from the United States, Europe and South America also offered testimony on global warming -- which along with modern slavery is at the top of Pope Francis's agenda -- ahead of a UN conference in Paris tasked with producing a historic climate pact.

"I hold the great hope that the Paris summit will reach an essential deal," the pope told the assembled speakers.

He urged the mayors to galvanise the desire for social and environmental change ahead of his address to the United Nations in September, warning that "if the message does not come from the ground up, it will not have any effect".

New York Mayor Bill de Blasio said that with the Paris summit just months away, "we need to see it as the finish line of a sprint, and take every local action we can in the coming months to maximise the chance that our national governments will act boldly."

"The pope has not invited us to ratify the status quo, but to put an end to it," he said, adding that his administration would work to reduce the city's carbon emissions by 40 percent by 2030.

New Orleans mayor Mitchell Landrieu described the moment Hurricane Katrina descended on his city, saying human greed had destroyed the city's defences: "For 100 years our coast has been slashed and burned for the benefit of the American oil consumer."

After the hurricane, "on the streets of America, a veil had been lifted to reveal an ugly, scary reality. It revealed that we are weak. It revealed that our problems from poverty to environmental degradation could destroy us all if we didn't change".

- 'Blind inertia' -

The governor of California, Edmund Brown Jr., said the mayors had to join forces and fight for change in the face of "fierce opposition and blind inertia," and despite the "hundreds of millions of dollars going into propaganda, into falsifying the science record".

Many argued that ills such as prostitution and human trafficking must be at the centre of efforts to deliver sustainable development.

"The environmental agenda is inseparable from the social agenda," Sao Paulo Mayor Fernando Haddad said.

The event, titled "Modern Slavery and Climate Change: The Commitment of the Cities", kicked off with Rome Mayor Ignazio Marino, who said "slavery still exists in our cities, even in Rome."

The former surgeon focused in particular on organ trafficking, saying the phenomenon was growing, with tens of thousands of operations carried out each year around the world to extract organs -- particularly kidneys -- for sale to rich patients.

The illegal operations are mainly carried out in China, India and Pakistan, he said, but warned that "Africa is the new frontier".