BRASILIA, June 22 (Reuters) - 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 to commercial agriculture and mining. Leaders of Brazil"s roughly 850,000 indigenous people say the bill is unconstitutional and runs roughshod over land rights guaranteed in the country"s 1988 Constitution. A session of the lower chamber"s constitutional affairs committee was suspended due to the incidents outside Congress. The committee"s chair, Bia Kicis, a supporter of far-right President Jair Bolsonaro, said on Twitter that "Indians linked to the left attacked the chamber" and wounded three police officers with arrows. One policeman needed medical attention, the chamber"s news agency reported. Indigenous land rights are also being debated by the Supreme Court, which is tasked with upholding constitutional guarantees. Among the provisions for which the bill in Congress would open the door are the building of hydroelectric dams and roads on reservations, and setting up of bases there by Brazil"s military that oppose indigenous reservations on the country"s borders. "This bill is a historical setback and has gained momentum under Bolsonaro who encourages the destruction of the rights of Brazil"s first peoples," said Almir Suruí, a tribal chief and head of Parlaindio, a forum for indigenous issues.