* President says power problems could last seven days
* Pinera pledges to fix transformer within 48 hrs (Adds quotes, details)
SANTIAGO, March 15 (Reuters) - Chile's main power grid will remain "unstable" for the next seven days, President Sebastian Pinera said on Monday, a day after a massive outage left most the world's top copper producer in the dark.
Millions of Chileans were left without power on Sunday evening after a transformer failed in the electricity grid, two weeks after a devastating earthquake killed hundreds and mangled roads and bridges in the South American country.
"We're committed to repair the damaged transformer within the next 48 hours and the lines that may have been affected should be checked and repaired within the next seven days," Pinera said.
"As it was the case following the earthquake on Feb. 27, in the next seven days we're going to have an unstable power system," he added.
Electricity was gradually restored on Sunday evening, but the failure of the South American country's main power grid is another test for conservative Pinera, who took office last week on pledges to rebuild Chile after the huge earthquake, which caused an estimated $20 billion to $30 billion in damage. (Reporting by Antonio de la Jara; Writing by Eduardo Garcia; Editing by Jackie Frank)