Three years ago, Pope Francis shattered the all-time record for turnout for a papal Mass by drawing an estimated six million people in Manila, the capital of the Philippines. The previous record had also been set in the heavily Catholic Asian nation, when St. Pope John Paul II is believed to have celebrated a concluding Mass at World Youth Day for somewhere between four and five million.

Yet neither of those events are even close to being the Philippines’ biggest religious gathering. That pride of place belongs to the annual Black Nazarene procession, and the Asian Catholic news agency UCAN reports that this year, the week-long celebration is expected to draw close to 20 million people to Manila.

The apex will come on Tuesday, when a feast known as the Translacion, celebrating the passage of a sacred image from one location to another, is marked.

Every year on Jan. 9, millions gather in Manila for a procession of the Poong itim na Nazareno, a life-sized statue of a suffering Jesus fallen under the weight of the Cross. It’s held along a three-mile route from Rizal Park, where Francis delivered his “Thrilla in Manila” in 2015, to the Minor Basilica of the Black Nazarene, in the district of Quiapo, where the image is kept throughout the year.

This time around, organizers are echoing the environmental-friendly message Francis delivered in the Philippines and throughout his pontificate, urging people to keep the celebration “trash-less.” The appeal has been made by several environmental groups and Father Douglas Badong, vicar of the Minor Basilica of the Black Nazarene in Quiapo district.

“Let us consider our environment in our expression of faith,” the priest said.

According to UCAN, Monsignor Hernando Coronel, rector of the basilica, also called on Catholics “to translate the devotion into actions that would serve the community.”

“Let us become ecological stewards like how we protect the image of the Black Nazarene during the procession,” he said.

Last year, over 300 tons of garbage were collected throughout the week along the route of the procession, some 70 tons after the 20-hour pilgrimage from the park to the church. These statistics led to last week’s call from several pro-environment groups, urging Black Nazarene devotees to renounce littering and maintain a “zero-waste” policy during the procession.

The suggestions — made by the EcoWaste Coalition, Buklod Tao, and the Green Brigade Committee of the Quiapo Church — include asking for pilgrims to refrain from smoking or vaping as an “act of penance,” to return used beverages and food containers to givers or vendors for proper disposal, and to store their food in reusable cloth bags instead of plastic ones.

In recent years, among the most littered items were food packaging, plastic cups, food leftovers, bamboo skewers, PET bottles, plastic bags, newspapers and cigarette filters.

Speaking from experience, the group also asked for people not to relieve themselves in PET bottles, nor in the walls of Rizal Park or the streets, saying that it’s “unsightly and unsanitary.”

The environmental groups asked the devotees to “express their faith in a manner that is respectful of the environment.”

“The Catholic devotion to the Black Nazarene is truly breathtaking and splendid. Sadly the annual Traslacion is sullied by the unrestrained littering at the Pahalik and vigil site in Luneta and along the processional route,” said Daniel Alejandre, of the EcoWaste Coalition in a statement.

The EcoWaste Coalition, self-defined as a public interest network of community, church, school, environmental and health groups pursuing sustainable solutions to waste, climate change and chemical issues, issued its own statement, fearing a repeat “of the heaps of garbage” left by the faithful on Dec. 31- another big day for the Black Nazarene devotees, since it marks the beginning of the novena in his honor.

“As a show of reverence to the Black Nazarene, we request the faithful to manifest their solemn devotion in a way that will not overwhelm Rizal Park and the processional route with litter,” he said.

“Cleaning up the mess left behind by the devotees can be a grueling task for government workers and for volunteers from various parishes, schools and groups,” he added.

According to the EcoWast Coalition, the protection and preservation of the city’s environment and God’s creation should also be “at the core” of the devotion to the Black Nazarene.

During off-the-cuff remarks at the University of St. Thomas in Manila three years ago, Francis urged youth to protect the environment, “not only because this country, more than many others, is likely to be seriously affected by climate change.”

The pope had a set of prepared remarks, which he decided to set aside – as he often did during this trip – opting instead to speak in Spanish through an interpreter. This decision was partially motivated by the fact that the Argentine pontiff was visibly moved by the questions posed to him, particularly that of a 12-year old girl named Glyzelle Palomar, who asked  the pope, “Why do children suffer?”

However, Francis requested for his original speech to be published. In it, he focused even more on the environment, something that was expected: Preparations for the papal visit were partially set in motion after a 2013 super-typhoon in the central Philippines left 6,000 dead and 4.1 million homeless.

“You are called to care for creation not only as responsible citizens, but also as followers of Christ!” the pope wrote in the speech, available on the Vatican’s website. “Respect for the environment means more than simply using cleaner products or recycling what we use. These are important aspects, but not enough.”

He went on to say that by destroying forests, ravaging the soil and polluting the seas, humanity betrays the noble calling of being “stewards of God’s creation.”

Francis insisted on this during his homily in the Mass in Rizal Park that drew a crowd estimated at six million people.

“[God] created the world as a beautiful garden and asked us to care for it,” he said. “But through sin, man has disfigured that natural beauty; through sin, man has also destroyed the unity and beauty of our human family, creating social structures which perpetuate poverty, ignorance and corruption.”

The actual feast of the Black Nazarene is marked on Good Friday. However, this week’s procession is even more popular. It reenacts a seemingly minor historical event from 1787, known as the solemn Translacion, meaning transfer, of the image from its original home, where Rizal Park is now located, to its present home at the basilica in Quiapo.

Most Filipino Catholics consider the Nazarene statue to be miraculous, able to heal terminal cancers and other sicknesses, to grant petitions, and to help those in need.