Experts described the procedure as generally low risk and almost always having positive results. But the surgery on the 60-year-old, which involves drilling into the skull, worried many Argentines, who have struggled to imagine their country with anyone else at its center.
The website of the Argentine presidency announced early Tuesday that the surgery had begun, but hours later there was no official word on her progress. The hospital was sealed off, and all inquiries were referred to presidential spokesman Alfredo Scoccimarro, who did not answer his phones.
Dozens of her supporters gathered outside the Fundacion Favaloro, one of Argentina's top cardiology hospitals, after keeping vigil there the night before. Some carried signs with messages such as "Fuerza Cristina," urging her to show her strength.
Fernandez was diagnosed with "chronic subdural hematoma," or fluid trapped between the skull and brain. This can happen when the tiny veins that connect the brain's surface with its outermost covering, or dura, tear and leak blood. As people age, it can happen with a head injury so mild that they don't remember it.
In the president's case, doctors initially prescribed a month's rest, because in some cases the fluid can be absorbed without intervention, but they decided on surgery after she complained of numbness and weakness in her upper left arm, in addition to the headaches and irregular heartbeats she has been suffering.
While messages of sympathy poured in, critics questioned the secrecy surrounding her health. Her condition was announced in a three-paragraph statement late Saturday after she spent more than nine hours in the hospital. It said she suffered a "traumatismo cranial" on Aug. 12, but gave no details on how this injury happened.
August 11 was a rough day for the president. Despite her intensive campaigning, primary election results that night showed a significant drop in support for her party's candidates ahead of the Oct. 27 congressional elections.
Fernandez, who followed her highly popular husband into the presidency, has come to dominate Argentine politics after nearly six years in office, and now she'll be off the campaign trail just three weeks before voting day.
Vice President Amado Boudou was put in charge during her surgery, but there was no official announcement of the handover of power, nor were any documents released formalizing Boudou's new role, but government websites eventually began describing him as "the vice president in charge of the executive branch."
Boudou, for his part, took Fernandez's spot behind the podium in the Casa Rosada and told top officials in a televised address that he would run the country as a team "while she gets the rest she deserves."
"What Cristina wants is for us to maintain the administration, and to carry on this project that (her late husband) Nestor Kirchner began and that Cristina has continued," said Boudou, whose popularity has sunk amid ongoing corruption investigations.
Argentina's constitution provides for, but does not require, a formal transfer of power in case of extended health problems, said Daniel Sabsay, a constitutional lawyer. A full medical leave would require congressional approval, but short of that, "she alone decides, according to the problem she faces and her doctors' advice, if she needs to delegate some powers to the vice president," he told Radio Continental.
"There needs to be more information to lower the people's anxiety," said Fabian Perechodnik, who directs the Poliarquia political consulting firm.
Associated Press writer Almudena Calatrava contributed to this report.