* Center-left president diagnosed with thyroid cancer

* Doctors say she has good chance of full recovery

* Upbeat Fernandez vows to carry on "with same energy"

Argentina's President Cristina Fernandez de Kirchner gestures next to an image of former first lady Eva Peron during a ceremony at the Casa Rosada Presidential Palace in Buenos Aires, December 28, 2011. [Photo/Agencies]

BUENOS AIRES  - Argentine President Cristina Fernandez, who will have cancer surgery next week, called for moderation from unions and big business on Wednesday in her first appearance since her illness was made public.

Fernandez, 58, has a papillary carcinoma, the most common form of thyroid cancer, and there is no sign the disease has spread. Doctors say she has a very high chance of recovery and will not need chemotherapy or radiotherapy. The diagnosis was made public on Tuesday.    

"I want to ask all of you for your help, not for my sake, but for the country," she said, urging union leaders and company bosses to exercise "prudence and balance" as double-digit inflation fuels wage demands and erodes the competitiveness of Argentine-made products.

Fernandez's cancer scare comes less than a month since she was sworn in for a second four-year term after winning a landslide re-election in October. She is due to have surgery on January 4 and take 20 days leave afterward.

"We're going to carry on with the same energy as ever," Fernandez said.

Her already high approval ratings could get a further boost from public sympathy over her illness as they did following the death late last year of her husband and predecessor as president, Nestor Kirchner.

At a time of heightened tensions with the country's most powerful union leader, Hugo Moyano, it might also help quell potential labor unrest.  

Moyano controls the key truckers union, which has been pushing companies for an end-of-year bonus in recent days.

"This isn't a government issue, but anyway, there could be a truce if the president isn't there," Hugo Pistone, a spokesman for the truckers, told local radio.

Argentine financial markets took news of the president's cancer diagnosis calmly and the peso currency was virtually unchanged in late morning trade.

"We don't expect the news to have an impact on markets, owing to the good prognosis on the president's health," Barclays Capital analyst Sebastian Vargas wrote in a briefing note.

Fernandez's health has been in the spotlight before.

The former senator suffers low blood pressure, and some commentators suggested earlier this year that she might not run for re-election due to health concerns and pressure from her family following Kirchner's death of a heart attack.

When Kirchner died, many thought it spelled the end of the couple's idiosyncratic blend of state intervention, nationalist rhetoric and the championing of human rights.

But Fernandez's popularity rebounded on the back of a brisk expansion in Latin America's No 3 economy, public sympathy and the opposition's failure to mount a convincing challenge.

Fernandez, a skilled orator fond of glamorous clothes and high heels, is one of several regional leaders to have cancer.  

Venezuelan President Hugo Chavez and Paraguay's Fernando Lugo both underwent chemotherapy recently while former Brazilian leader Luiz Inacio Lula da Silva is currently being treated for throat cancer.