Argentina's President Cristina Fernandez de Kirchner underwent successful surgery to remove a blood clot close to her brain. She is recovering in an intensive care unit, doctors from the Favaloro Foundation clinic in Buenos Aires said.
"The operation was a success and she's in good spirits and accompanied by her family," her spokesman Alfredo Scoccimarro said, without providing further details. The next bulletin on her health will be released today.
Doctors, who initially advised a month's rest, decided to operate when Fernandez subsequently complained of tingling in her left arm. Since her October 2011 re-election, the lawyer and former senator has cancelled activities, including trips abroad, at least four times because of low blood pressure.
Vice-President Amado Boudou yesterday assumed Fernandez's duties, as he did when she underwent thyroid surgery in January 2012 for a misdiagnosed cancer.
Experts described the procedure Fernandez underwent as generally low-risk and almost always successful. But the surgery involved drilling through the 60-year-old leader's skull, and recovery can take months, worrying many Argentines ahead of a crucial mid-term election.
Hundreds of supporters cheered and chanted at word she was recovering from the operation. Some had kept vigil through the night outside the Fundacion Favaloro, carrying signs, statues of Argentina's virgin and messages such as "Fuerza Cristina", urging her to show her strength.
The hospital was sealed off, and all inquiries were referred to Scoccimarro, who initially did not answer his phones, prompting critics to question the secrecy that has surrounded Fernandez' health.
Her condition was announced in a three-paragraph statement late on Saturday after she spent more than nine hours in hospital, attributing the injury to a blow to her head on August 12. That would have been the day after primary elections showed a sharp drop in support for her party's congressional candidates despite her campaigning.
The statement gave no details about how the injury happened.
As Fernandez returned to the hospital on Monday in preparation for surgery, Boudou said that top officials would run the country as a team "while she gets the rest she deserves".
"There needs to be more information to lower the people's anxiety," said Fabian Perechodnik, who heads the Poliarquia political consulting firm.
Opposition candidate Jose Ignacio de Mendiguren also raised concerns.
"We should be getting more information about the seriousness of the issue," he said.
Additional reporting by Associated Press, Agence France-Presse