A slightly frail-looking Gandhi appeared at a memorial to independence hero Mahatma Gandhi and paid floral tributes on his birth anniversary. She walked in her usual brisk pace and showed little sign of any discomfort.

Gandhi also participated in a brief prayer meeting with other political leaders, television images showed.

The 64-year-old Gandhi's absence coincided with a plunge in popularity for the centre-left party, buffeted by the biggest anti-corruption protests since the 1970s, high inflation and challenges from the opposition that frequently halted parliament proceedings.

Gandhi returned to India in early September after the surgery and chaired a closed-door party meeting in the middle of last month.

The Indian media has largely stayed away from covering her illness and no images of her had been released by her party since she slipped out of the country in early August for an operation.

Neither Gandhi nor her party have disclosed the nature of her illness or how serious it was, but several media reports said she was operated on for cancer at a specialist hospital in New York.

The Gandhi family, descended from India's first Prime Minister Jawaharlal Nehru, enjoys a status similar to royalty in the country of 1.2 billion. They are not related to independence hero Mahatma Gandhi, a close ally of Nehru.

Italian-born Sonia was married to Rajiv Gandhi, Nehru's grandson and a former prime minister, who was killed by a suicide bomber in 1991 while campaigning for elections. His mother, Indira Gandhi, was also prime minister when she was assassinated by her Sikh bodyguards in 1984.

Sonia's son, Rahul, is widely expected to be the next prime minister if the Congress party returns to power in the next 2014 general elections.