Sonia Gandhi, the president of India's ruling Congress party who is widely seen as the country's most powerful politician, was taken to hospital on Monday after feeling unwell during a marathon parliament debate on a major new food welfare scheme.
Gandhi was led limping out of the lower house in the early evening by her son and colleagues, then taken by car to the All India Institute of Medical Sciences (AIIMS) hospital in New Delhi.
"Sonia has been admitted to AIIMS, her condition is stable," party spokesman Bhakta Charan Das told Reuters. He did not give more details.
It was not immediately clear what caused the turn six hours into the debate. Some media reports said she had complained of chest pain, others that she was suffering from a viral fever. A spokesman at the hospital could not be reached for comment.
The Italian-born politician, who has led the party to two successive terms governing the world's largest democracy, has played a slightly less public role in politics since being treated abroad for an unknown illness in 2011. The party is usually very secretive about her health.
Gandhi, 66, became Congress chief some years after the assassination in 1991 of her husband, the former Prime Minister Rajiv Gandhi.
She declined to become prime minister despite pressure from the party after the first victory in 2004, and chose instead the quiet economist Manmohan Singh for the top job.
However, she arguably wields more power over government policy than Singh from her post as Congress president, with party members and cabinet ministers grateful to her for reviving their fortunes with two uninterrupted terms in office.
Earlier on Monday, television images showed the normally strong-looking leader trembling as she read out from a clutch of papers a short speech on the bill, which is a flagship scheme for the government and a pet project of Gandhi.
"I am standing in support of food bill. It's time to take the historic step," she said. "It is my fervent appeal that we shall pass this unanimously."
Gandhi had made it the government's top legislative priority before the next election to pass the bill that gives subsidised grain to almost two-thirds of India's 1.2 billion population. It was passed two hours after she and her son Rahul Gandhi left parliament and must now go to the upper house.
Rahul Gandhi is the scion of the family, one of the world's most successful modern political dynasties that stretches back to the nation's first prime minister, Jawaharlal Nehru.
The young Gandhi is leading the party's preparations for a national election - due to be held in less than a year.