Catholic leaders in the Amazon basin region have condemned Brazilian President Michel Temer's decree opening up a vast area of untouched rain-forest to mining companies.

The decree removed a previous ban on private companies exploiting a nature reserve, which covers more than 46,000 square kilometres in the northern Para and Amapa states.

President Temer said this was part of a programme to boost Brazil's weak economy and that vital areas within the reserve will remain protected.

The Pan-Amazon Ecclesial Network, a church body covering the region, said it joined other Catholic organisations and indigenous tribes in the targeted area of northern Brazil to "publicly reject the anti-democratic announcement" scrapping a vast nature reserve on 23 August.

"The opening of the area for mineral exploration of copper, gold, diamond, iron, niobium, among others, will increase deforestation, irreparable loss of biodiversity and negative impacts on people throughout the region," a statement said.

The Pan-Amazon Ecclesial Network joins Greenpeace and the WWF in condemning the decision as giving in to pressure from big business, while sacrificing the so-called "lungs of the world".

"It is enough to observe the destruction that Brazilian and foreign miners have left in the Amazon in the last decades, with deforestation, pollution and water resources being compromised by the high consumption of water for mining," the church statement said.

It also predicted that mining companies would bring "increased violence, drugs and prostitution, escalation of conflicts over land, [and] uncontrolled aggression against the cultures and lifestyles of indigenous and traditional communities."