Nand Kumar Patel, the head of India's Congress Party, is among at least 27 people who were killed in a Maoist ambush in Chhattisgarh State.
Former state home minister Mahendra Karma, who had set up a controversial anti-Maoist group in 2005, was also killed.
The group of around 300 rebels targeted the Congress Party convoy in Sukma district, about 345 kilometers south of Raipur, as it was leaving the area following a political rally Saturday night.
The attackers triggered a land-mine before opening fire at the group. At least five policemen also lost their lives.
At least 32 others were wounded in the attack, which is being blamed on Naxal rebels who have been fighting the Indian government for years in demands for better treatment of tribal groups in the country.
President of the Congress Party Sonia Gandhi has said she is "devastated" by the attacks, which also killed Patel's son Dinesh.
Prime Minister Manmohan Singh called the attack "barbaric," and said that India would "never bow down" to the rebels, BBC News reported.
"It is not an attack on Congress," said Congress Vice President Rahul Gandhi. "It is an attack on democracy. But, we’ll not fear from such an attack and continue to move forward with enthusiasm."
The Naxals are estimated to have 6,000 and 8,000 militant fighters in nearly a third of India's 630 districts. The Maoist rebels say they are fighting for the rights of India's poor farmers and landless laborers.
They have killed thousands since their insurgency began in the late 1960s.
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