"We can confirm 17 deaths. Among those who have been killed are five policemen. The rest are all leaders of the state Congress unit," Rajinder Kumar Vij, the chief of anti-Maoist operations of Chhattisgarh state, told AFP.
The whereabouts of state Congress chief Nand Kumar Patel and his son, who were abducted during the attack, was not known, Vij added.
Former cabinet minister Vidya Charan Shukla was fighting for life after being wounded in the attack, carried out in a thickly forested area of Jagdalpur district, 284 kilometres (176 miles) from state capital Raipur.
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The rebels detonated a bomb before firing indiscriminately at the convoy, which was returning from a political rally nearby, police said.
The attack is the latest in a simmering conflict that pits the left-wing insurgents against local and national authorities in the forests and rural areas of mainly central and eastern India.
The guerrillas, who say they are fighting for the rights of tribal people and landless farmers, often collect funds through extortion and protection rackets.
Congress chief Sonia Gandhi condemned the incident Saturday as "shocking" and said the party was pained by the attack on its colleagues.
"Naturally we are devastated... It is despicable that ordinary people engaged in political activity were attacked," she told reporters in New Delhi after an emergency meeting with Prime Minister Manmohan Singh.
Singh, who has described the Maoists as the country's most serious internal security threat, spoke to state Chief Minister Raman Singh and promised him all "necessary aid".
"Government will take firm action against the perpetrators of violence of any kind," he said in a statement.
In 2009, government forces launched a huge anti-Maoist offensive known as "Operation Greenhunt", but the often poorly-trained police have had to contend with a deadly series of attacks.
Three policemen were killed a fortnight ago when Maoists launched an overnight attack on a state-run broadcaster in Chhattisgarh.
And last week a policeman and eight villagers were killed in a shootout between rebels and security forces.
Eleven policemen were killed in March last year in a landmine blast in western Maharashtra state close to the border with Chhattisgarh
