Indigenous leaders in Brazil have denounced Jair Bolsonaro’s “colonialist and ethnocidal” policies as the far-right populist headed to New York to defend his treatment of the Amazon and its inhabitants.

Bolsonaro is set to make the opening speech at the UN general assembly on Tuesday morning after a wretched few weeks for Brazil’s international reputation in which reports of soaring deforestation and his response to the Amazon fires have cemented his reputation as South America’s “Captain Chainsaw”.

Brazil’s president is expected to use his UN debut to launch a Trumpian assault on the left and push back against foreign criticism of his treatment of Brazil’s environment and indigenous communities. He has recruited a rare pro-Bolsonaro indigenous voice, Ysani Kalapalo, to travel with him to New York in an effort to soften his notoriety as a rainforest destroyer.

But in a strongly worded open letter, 16 indigenous leaders from Brazil’s Xingu indigenous park spurned Bolsonaro’s “colonialist and ethnocidal” programme for their communities, which he has pledged to open for commercial exploitation.

The leaders claimed the sole interest of Kalapalo – who recorded a recent video denying Bolsonaro was to blame for the Amazon fires – was to “insult and demoralize Brazil’s indigenous leaders and movement” on social media.

“Not content with its attacks on indigenous peoples, the Brazilian government now seeks to legitimize its anti-indigenous policies by using an indigenous figure who sympathizes with its radical ideologies,” they added.

In a recent article for the Guardian, the Xingu’s best-known leader, Raoni Metuktire, accused Bolsonaro of doing nothing to stop ranchers invading indigenous lands.

Bolsonaro’s UN appearance will represent the culmination of a government propaganda drive designed to repair Brazil’s global image and fend off the threat of economic sanctions.

“We are not the villain of the environment,” Bolsonaro’s environment minister, Ricardo Salles, insisted during an interview in New York.

But observers are awaiting Bolsonaro’s speech – which one commentator expected to focus on the issues of “sovereignty, liberalism, communism/left, Christianity and the Amazon” – with considerable trepidation.

In January, Bolsonaro’s unusually brief international debut at the World Economic Forum in Davos was widely panned as well as being overshadowed by a snowballing scandal linking one of his politician sons to Rio gangsters.

Writing in Rio’s O Globo newspaper, the former head of Brazil’s foreign service, Marcos Azambuja, fretted that Bolsonaro’s address would showcase a new-look Brazil that has shocked the world: a place of narrow sectarianism, religious zeal and naive and foolhardy diplomacy.

“I have no advice for whoever is preparing our speech. I’d just like to recall the … warning once given to two inexperienced hedgehogs about how to make love,” Azambuja wrote. “Be very careful.”