Brazilian indigenous activists are expected to protest in the capital Brasilia on Wednesday ahead of a Supreme Court verdict.
At least 2,000 tribal representatives from around the country are to demonstrate against what they see as plans to hand over their lands to the powerful agricultural industry.
Their rally is raising concerns of the possibility of a repeat of violent clashes in April at which riot police fired tear gas at tribesmen in traditional headgear and paint -- and armed with bows and arrows -- outside Congress.
Brazil's 1988 constitution guarantees tribes ownership of ancestral lands. But under a proposal being studied by the Supreme Court, the guarantee would not apply to land unoccupied prior to the law coming into effect that year.
Activists say that makes no sense given the reality of life in sparsely inhabited territory claimed by tribes as part of their heritage -- and eyed by the agribusiness lobby as a place to expand soy, cattle, sugar cane and other commodity farming.
"It's a very serious interpretation that will limit the indigenous peoples' rights," Juliana de Paula Batista, a lawyer with the Instituto Socioambiental, a group defending indigenous rights, told AFP.
"Some of them weren't occupying their lands in 1988 because they are nomadic or because they had been expelled previously under the military dictatorship."
There is uncertainty over what direct impact a court decision against the tribal representatives would have.
The 114 demarcation cases currently under analysis would likely be affected, but there is a question over whether a decision could open the door to many more territorial challenges.
"We're living in a time of worsening violence on the ground and today there is a far more insidious policy regarding indigenous rights, reinforcing the hand of the agricultural lobby in Congress," de Paula Batista said.
Nearly 900,000 indigenous tribe members currently live in Brazil, or 0.4 percent of the entire population, divided into 305 ethnic groups. Indigenous lands cover 12 percent of Brazil.
Much of that is in the Amazon rainforest region. But land that the outside world sees as one of the planet's greatest natural wonders is in the crosshairs of an ever expanding agricultural industry.
And while the government is committed to protecting those lands, the fact that many of the borders are not yet officially demarcated effectively deprives the tribal members of rights, resulting in constant legal and physical pressure.
In May, President Michel Temer, who has ties to the agribusiness lobby, sacked the head of the agency dealing with indigenous issues, FUNAI.
The ousted president of FUNAI, Antonio Fernandes Toninho Costa, claimed he was fired because he had been too strong in his defense of tribal rights.
Brazil's management of its environmental riches will also come under scrutiny at the Supreme Court on Wednesday in a possible ruling on the system for creating conservation areas around the country.