The president of India's Congress Party, Sonia Gandhi, has said she is "devastated" by Saturday's attack on party officials in Chhattisgarh state.
At least 23 people were killed, including Chhattisgarh Congress chief Nandkumar Patel, his son, and local leader Mahendra Karma, when suspected Maoist rebels ambushed their convoy.
Mrs Gandhi denounced the "dastardly attack" on India's democratic values.
She and Prime Minister Manmohan Singh are to visit Chhattisgarh on Sunday.
Her son, Rahul, the vice-president of the Congress party, has already met survivors at a hospital in the state capital, Raipur.
The BBC reports the convoy carrying state Congress leaders and party workers was ambushed at around 5.30pm on Saturday as it travelled through the Darba Ghati valley, in the Sukma area about 345km south of Raipur, following a campaign rally.
Police said the convoy was first forced to stop by felled trees on the road. At least one vehicle at the front of the convoy was then hit by a land-mine blast before as many as 200 suspected rebels opened fire, they added.
The main target of the attack is believed to have been Nandkumar Patel, who witnesses said jumped into a ditch with his son, Dinesh, to take shelter before they were abducted.
Their bodies were later found nearby; the Congress chief was reportedly mutilated.
Mr Karma was found dead at the scene. A former home minister in Chhattisgarh, he founded the Salwa Judum, a local militia given the task of fighting the Maoists which has been accused of abuses.
Police said 32 people were wounded in the ambush, many of them seriously.