Members of the Gamela tribe suffered gunshot wounds, stabbings and amputations in a land rights dispute in the Amazon state of Maranhao.
Reports say the attackers were farmers and landowners.
There has been a rise in such assaults across Brazil, with the perpetrators rarely caught.
Human rights groups say proposed government cuts in the budgets of environmental enforcement agencies will worsen the situation.
According to reports, the Gamela tribal camp was surrounded by farmers and landowners armed with clubs, guns and knives.
Several tribespeople were shot and stabbed - and at least two had their hands hacked off with machetes.
The severely wounded victims, who numbered at least a dozen, were taken to hospital in the state capital, Sao Luis.
The impoverished and violent north-eastern state of Maranhao is at the forefront of the fight against illegal deforestation and encroachment on indigenous lands, the BBC's Wyre Davies says.
But he adds that the perpetrators of attacks on tribal people are often protected by corrupt politicians and police allied to landowners.
Indigenous leaders who try to take legal action, and their supporters in environmental pressure groups, are often deliberately targeted or even assassinated in a country where rural violence often goes unpunished.
Human rights groups in Brazil have called on the government to increase protection for indigenous people after a tribe was attacked.