Pope Francis has urged Roman Catholic Church leaders to make bold changes to better protect the Amazon and its indigenous people at the start of a three-week meeting that could become one of the most controversial moments in his papacy.

The Pope celebrated an opening Mass in St Peter's Basilica on Sunday, where he criticised a “new colonialism” that puts profit ahead of protecting the environment.

He prayed that God's "daring prudence" would inspire religious representatives to spread the Catholic faith while remaining open to change.

"If everything continues as it was, if we spend our days content that 'this is the way things have always been done,' then the gift vanishes, smothered by the ashes of fear and concern for defending the status quo," he said.

One of the most contentious topics of the synod, whose 260 participants are mostly bishops from the Amazon, is whether to allow married men considered leaders in their local communities to become priests.

It has been proposed to tackle an acute shortage of priests in isolated regions in order to allow Catholics to receive the sacraments regularly. At least 85 per cent of villages in the Amazon, a vast region that spans eight countries and the French territory of Guiana, cannot celebrate Mass every week. Some see a priest only once a year.