Environmentalists are demanding urgent action to halt an aquatic gold rush along one of the Amazon River’s largest tributaries, where hundreds of illegal goldmining dredges have converged in search of the precious metal.

The vast flotilla – so large one local website compared it to a floating neighbourhood – reportedly began forming on the Madeira River earlier this month after rumours that a large gold deposit had been found in the vicinity.

“They’re making a gram of gold an hour down there,” one prospector claims in an audio recording obtained by the Estado de Sao Paulo newspaper.

Danicley Aguiar, an Amazon-based Greenpeace activist who flew over the mining flotilla on Tuesday, said he had been stunned by the magnitude of the illegal operation unfolding just 75 miles east of the city of Manaus.

“We’ve seen this kind of thing before in other places – but not on this scale,” Aguiar said of the hundreds of rafts he saw hoovering up the Madeira’s riverbed near the towns of Autazes and Nova Olinda do Norte.

“It’s like a condominium of mining dredges … occupying pretty much the whole river.”

Aguiar added: “I’ve been working in the Amazon for 25 years. I was born here and I’ve seen many terrible things: so much destruction, so much deforestation, so many illegal mines. But when you see a scene like that it makes you feel as though the Amazon has been thrust into this spiral of free-for-all. There are no rules. It’s as if we’re living in Mad Max.”

There was outrage as footage of the riverine gold rush spread on social media.

“Just look at the audacity of these criminals. The extent of the impunity,” tweeted Sonia Bridi, a celebrated Brazilian journalist known for her coverage of the Amazon.

Andre Borges, another journalist whose story helped expose the mining flotilla, tweeted: “We are witnessing, in 2021, a goldminers’ uprising with all the aggressiveness of the days of discovery.”

Brazil’s multimillion-dollar illegal mining industry has intensified since the 2018 election of Jair Bolsonaro, a far-right nationalist who backs the wildcat garimpeiros who trawl the Amazon’s rivers and rainforests for gold.

As many as 20,0000 garimpeiros are believed to be operating within the supposedly protected Yanomami indigenous reserve in Roraima, one of nine states that makes up the Brazilian Amazon.

Deforestation has also soared under Bolsonaro, who has stripped back environmental protections and been accused of encouraging environmental criminals. Amazon destruction rose to its highest levels in 15 years between 2020 and 2021 when an area more than half the size of Wales was lost.

Last week the Bolsonaro administration was accused of deliberately withholding new government data laying bare the scale of the deforestation crisis to avoid international humiliation during the Cop climate summit, which Brazil’s president declined to attend.

Aguiar, a Greenpeace spokesperson for the Amazon, said Bolsonaro’s pro-development rhetoric was partly to blame for the gold rush playing out on the Madeira River. He also pointed the finger at regional politicians in the Amazon who supported plans to allow miners to exploit gold deposits in riverbeds.

In a recent interview, the former head of Brazil’s environmental agency Ibama, Suely Araujo, said she saw only one way of saving her country’s environment: electing a different president.

“It’s hard to believe that this government is going to look after the environment because they are destroying everything,” said Araujo, a public policy specialist for the Observatorio do Clima environmental group.