Riot police used tear gas and rubber bullets to disperse a group of indigenous people protesting on Tuesday outside Brazil's Congress against a bill lawmakers were about to debate that would undermine recognition of protected reservation lands.

The protesters, among them children and elderly people, ran for cover, many coughing their way through clouds of gas.

The bill, known as PL 490 and backed by Brazil's powerful farm caucus in Congress, is aimed at curtailing indigenous rights to land that was still unoccupied as of 1988. It also seeks to open up protected indigenous lands...