Gandhi was in Varanasi, the constituency of rival politician and Indian Prime Minister Narendra Modi in Uttar Pradesh state, when she suddenly pulled out to fly back New Delhi.
"I regret that I couldn't complete my trip because of illness," Gandhi said in a statement tweeted by her party.
"On advice of doctors she had to cut short..but she'll be back," said another Congress tweet.
The party made no official comment about Gandhi's medical condition but local news channels said she was suffering from a high viral fever and dehydration.
Reports added that she would be taken from the airport directly to a hospital in the capital for treatment and recovery.
Italian-born Gandhi, 69, is the widow of assassinated former premier Rajiv Gandhi. She leads a party that lost the 2014 elections to the right-wing Bharatiya Janata Party after consecutive wins in 2004 and 2009.
"Heard about Sonia ji's ill health during her Varanasi visit today. I pray for her quick recovery and good health," Modi said on Twitter.
The Gandhi dynasty, which has no link to independence hero Mahatma Gandhi, has exerted huge influence in India during most of its history since independence with three of its members holding the office of prime minister.
Indian Congress Party President Sonia Gandhi receives floral tributes during a roadshow in Varanasi on August 2, 2016, ahead of 2017 Uttar Pradesh State Assembly elections. AFP
Sonia Gandhi, the head of India's opposition Congress party, fell ill at an election campaign event Tuesday.
