Brazilian officials served up an array of plans and figures at the recent COP28 climate summit in Dubai, presenting itself as a world leader, on track to protect its forests and the people who live there.

But on Thursday, Brazil’s Congress approved a law that threatens Indigenous people’s rights to most of the land they inhabit or claim, potentially opening vast territories to deforestation, farming and mining.

The new law requires that Indigenous people must provide concrete evidence that they occupied the land they claim on Oct. 5, 1988, when the country’s current Constitution was enacted — a requirement that many of them have little or no hope of meeting.

Under the new rule, not only can Indigenous land claims currently going through the legal process be thrown out for lacking such documentation, but established legal protections for Indigenous territories can also be challenged in court and rescinded.

“We have watched the entire world at COP28 saying that we need to change the direction the planet is taking,” said the leftist congressman Tarcísio Motta, who voted against the bill, “but congress has just withdrawn the rights from the people who point to the future of the planet.”

Studies have repeatedly shown that protected Indigenous territories have helped prevent Amazon deforestation, meaning the forest can better store carbon to fight climate change.

In September, Brazil’s Supreme Court ruled against a 1988 cutoff date for Indigenous land claims, but backers of the new law, who include powerful agricultural interests, hope it will change the legal calculus.

The Congress passed the legislation last month, but President Luiz Inácio Lula da Silva quickly vetoed most of its provisions. Then on Thursday, the House and Senate overrode the president’s veto, with many of his own allies joining his opponents in voting to defy him. Lawmakers also recently passed a measure that environmentalists call the “poison bill,” which relaxes rules on pesticides, and sent it to the president.

The Congress “has agreed with the agenda of the agribusiness caucus and of environmental setbacks,” said Marcio Astrini, the executive-secretary of the Climate Observatory, a network of environmental and civil society organizations in Brazil.

The law on Indigenous lands is expected to take effect by next week. Legal experts expect it to be challenged in the Supreme Court, and members of Apib, a leading Indigenous rights movement in Brazil, have already prepared a request for the court to review it.

Yet it could take months or more for the court to rule on the case, and environmentalists and Indigenous rights activists fear the harm that could be done by then.

“We will watch total chaos in jurisprudence and threats to the lives of these vulnerable people who depend on state action and on these territories to survive”, said Beto Marubo, an Indigenous leader and advocate of Indigenous rights from the Javari Valley of the Amazon basin, home to some of the most isolated people in Brazil.

Brazil has more than 1.7 million Indigenous people, according to official figures, and more than half live in the Amazon region. But only 20 percent of households with at least one Indigenous person live within designated Indigenous territories.

Those who do live in the territories already struggle against illegal forest-clearing for ranching and mining, and live with legal uncertainty, but the rate of deforestation is much lower in the Indigenous territories than elsewhere.

Across Brazil, 483 such territories have been granted full legal protection, and 278 others are going through the process to gain protection, according to FUNAI, a government agency.

Altogether, they cover more than 1.1 million square kilometers, or about 425,000 square miles, the size of Texas and California combined, almost 14 percent of Brazil’s area.

Advocacy groups say that under the new law, more than 90 percent of these lands could have protection lifted, and they have called out the government for undermining Mr. Lula’s environmental agenda, including preservation of the Amazon rainforest.

“It is a very contradictory situation for the country to have a policy to cut deforestation, and, on the other hand, have a Congress that fights tirelessly to end the richest instrument we have for protecting the Amazon: the Indigenous lands,” said Mr. Astrini.

Indigenous and environmental groups say that tribes with traditional lifestyles can have occupied an area for centuries without having any way to prove it. Some have had only passing contact with the developed world.

Congressmen who support the law argue it is needed to give landowners confidence that their land would not be taken from them, which would also create a better business environment for agriculture.

“What is happening today, with the overturning of the veto on the ‘time frame bill,’ is admirable because it brings legal certainty to those who own rural properties in Brazil,” said Márcio Bittar, a right-wing senator.

But it is the Indigenous whose land has been — and is being — taken from them, their advocates say, and the law ignores their history of dispossession and marginalization.

Outside of the government buildings in Brasília on Thursday, at least 100 Indigenous people and their supporters, including the government’s minister of the Indigenous Peoples, Sônia Guajajara, protested the bill as lawmakers inside voted to override the veto. Afterward, they headed to the nearby Supreme Court building to symbolically file their request for review.

Flávia Milhorance reported from Rio de Janeiro and Paulo Motoryn from Brasília.