Argentina's popular president, Cristina Fernandez, was successfully operated on for thyroid cancer on Wednesday, local media said without providing any further details.
The reports, however, were not yet confirmed by the government. The presidential spokesman was expected to address reporters outside the Austral Hospital where Fernandez underwent surgery in Pilar, some 28 miles (45 km) north of the capital Buenos Aires.
Last week, the government announced her diagnosis of papillary carcinoma, detected during a routine medical checkup just before Christmas but said there was no sign the disease had spread.
The cancer scare came just months after she was re-elected to a second four-year term. Doctors have said the 58-year-old president has a better than 90 percent chance of recovery.
Fernandez's already high approval ratings could get an extra lift from public sympathy over her illness as they did following the death late in 2010 of her husband and predecessor as president, Nestor Kirchner.
The news of her illness shook a country in which Eva Peron, the wife of former leader Juan Peron, has been revered for decades after dying of cancer at the age of 33. Like Evita, Fernandez is loved by some for her efforts on behalf of Argentina's poor.
The operation began at around 8:20 a.m. (1120 GMT).
Holding signs reading Strength Cristina, dozens of supporters, who rallied around Fernandez after Kirchner's death, gathered outside the hospital.
We started a vigil since yesterday in hopes that everything goes well. We're supporting our president. We're rallying behind her, said Rosa Aguirre, 50, a homemaker.
Vice President Amado Boudou, the former economy minister and a loyal Fernandez aide, assumed the presidency during Fernandez's 20-day leave of absence.
Widely popular among Argentines who benefit from her generous welfare spending, the president often gets bad marks from business leaders who say her interventions in the economy frighten off investment.
She is the latest of several left-leaning Latin American leaders to have cancer. Venezuelan President Hugo Chavez, who underwent chemotherapy last year, speculated after the Fernandez diagnosis that the U.S. empire may have developed a way to give the illness to its political rivals.
Paraguayan President Fernando Lugo's lymphatic cancer is in remission and former Brazilian leader Luiz Inacio Lula da Silva is being successfully treated for a tumour on his larynx.
VOTES, HIGH HEELS
A talented orator fond of glamorous clothes, high heels and make-up, Fernandez still wears black as she mourns Kirchner.
Many thought his death spelled the end of the couple's idiosyncratic blend of state intervention, nationalist rhetoric and championing of human rights. But Fernandez won re-election in a landslide in October, helped by brisk economic growth fuelled in part by hefty grain export revenues.
Having secured a second term with 54 percent of the vote, she promised to stay true to her policies despite complaints from Wall Street and international investors over her unorthodox approach to the economy.
(Additional reporting by Alejandro Lifschitz; Writing by Hugh Bronstein and Luis Andres Henao)