Sonia set for landslide by-election win

But sizzling temperatures kept people indoors and only 40 percent of some one million registered voters turned out in her home constituency of Rae Bareli in Uttar Pradesh state, electoral officials announced.

"It's the heat that kept people indoors," a Congress party spokesman said as the temperature touched 42 degrees Celsius (107 degrees Fahrenheit) in Rae Bareli, a bastion of India's Nehru-Gandhi dynasty.

The voting was marred by a solitary incident in which unidentified gunmen shot at a rival political activist, police said.

The Italian-born 59-year-old widow of assassinated former prime minister Rajiv Gandhi had won the seat in 2004 parliamentary elections that brought her party to power in a coalition government.