The coordinated fast by the 34 prisoners in various jails in southern Chile has put pressure on Pinera to review laws that date from the country's 1973-1990 dictatorship and have labeled the inmates as terrorists for crimes such as setting fire to timber shipments.
The hunger strikers want their terrorism convictions dropped.
"I want to ask all of those worried about the health of the protesters to help us end this hunger strike," said Pinera, adding that his government will send two bills next week to reform anti-terror legislation and the military justice system.
Under the current law, crimes deemed terrorism are subject to much harsher penalties and suspects can be tried in military courts if the case involves security forces.
The Mapuche, or "people of the land," have clashed for years with authorities and farmers over ancestral lands in southern Chile in an age-old conflict that is turning increasingly violent.
Courts have so far blocked attempts to force-feed the prisoners after at least three of them were taken to the hospital. Doctors say the inmates have lost between 33 to 40 pounds (15 to 18 kg) each and many are suffering cramps, low blood pressure and dizziness.
The Mapuche hunger strike has been compared to a fast by political prisoners in communist-ruled Cuba, which sparked fierce criticism of the Havana government by Chile's political elite and some Pinera allies.
The police and military have been accused by human rights groups of using excessive force against the indigenous, who have come under fire for sometimes violent protests in which they have burned crops and forestry companies' trucks and machinery.
Crippled by poverty, the Mapuche lost lands to the newly formed states of Argentina and Chile in the early 19th century after fending off Spanish colonizers for generations.
