Argentina's Vice President Amado Boudou assumed control of the country Monday, as President Cristina Fernandez was admitted to a hospital to undergo surgery to remove blood from inside her skull and to relieve pressure on her brain.
Her operation is scheduled to take place Tuesday in Buenos Aires.
On Saturday, President Fernandez's doctors discovered a subdural hematoma, which is blood that collected between her brain and skull. They ordered the 60-year-old president to take a month's rest, but decided to operate after her condition worsened Sunday, when she felt weakness and a lack of blood flow in her upper left arm.
The president's condition may be the result of hitting her head during a fall in August, although she was cleared by her doctors at the time.
President Fernandez has been campaigning for elections at the end of October that will determine control of Argentina's Congress.
President Fernandez, who has been in office since 2007, had her thyroid glands removed last year after she was diagnosed with cancer, although later tests indicated no cancer was present.
Her husband, former president Nestor Kirchner, died after a heart attack in 2010. The next presidential election is due in 2015.