The Indian Express reported 150-200 militants struck the party's convoy of about 20 vehicles on the Bastar-Sukma national highway in Jagdalpur district. State Congress President Nand Kumar Patel and his son Dinesh Patel were kidnapped, witnesses said.
State party leader Mahendra Karma, former lawmaker Uday Mudaliyar and another leader, Gopi Vadhwani, were among the dead, the newspaper said.
The New York Times reported five police officers were among those killed.
Officials had yet to confirm the attack.
Government official M.A. Ganpathy said about 20 people were injured in addition to those killed, the Express said.
"First a blast, then there was firing on the second, third and fourth vehicles," survivor Chandreshwar Chandrakar said. "They were young women and men. They surrounded us from all directions. Several leaders were killed on the spot."
At least three witnesses said Karma was killed.
"Maoists first asked who is Mahendra Karma ... and as he came out of the vehicle with his hands up, Maoists beat him up with their rifles and then fired 10-15 bullets at him," one unidentified witness said.
"It is not an attack on Congress. It is an attack on democracy," the Press Trust of India quoted Congress Vice President Rahul Gandhi as saying. "But we'll not fear from such an attack and continue to move forward with enthusiasm."