Several hundred indigenous Brazilians protested outside the Congress building in Brasilia on Wednesday against a bill backed by the country's farm sector that would limit recognition of reservation lands.

Protesters wearing feathered headdresses and body paint later tried to invade a downtown building to demand the ouster of the head of the government's indigenous affairs agency Funai for not defending their interests.

Riot police fired pepper spray to contain the crowd, and some arrows were shot at the glass windows of the building.

"This bill PL 490 is harmful not just to us, but to humanity," Sonia Guajajara, head of Brazil's largest indigenous umbrella organization APIB, told Reuters.

The legislation, first tabled in 2007, would fix 1988 as the cut-off date for the demarcation of reservation lands if they were not occupied at that time. The lower house is expected to vote on the bill in committee next week.

Brazil's Supreme Court must rule on a case that would set jurisprudence in the issue and is expected to uphold indigenous arguments that fixing the 1988 date violates constitutional guarantees to ancestral lands.

"The bill was rejected by the human rights commission in 2009. This is an unacceptable setback for indigenous rights and a constitutional absurdity," Brazil's only indigenous lawmaker Joenia Wapichana said in a congressional committee session.

The bill would open up protected indigenous lands to commercial agriculture and mining, and is backed by far-right President Jair Bolsonaro who advocates developing the Amazon region by tapping mineral resources on reservations.

Guajajara said the campaign to stop passage of the bill comes at a time of increased attacks on indigenous communities by wildcat gold miners who have illegally invaded indigenous lands.