Prime Minister Manmohan Singh and Congress chief Sonia Gandhi condemned the incident, which took place in Sukma district around 5.15 p.m. when the leaders were returning to Jagdalpur after holding a parivartan rally. The state is headed for assembly elections later this year.
Senior Congress leader and former leader of opposition Mahendra Karma and former legislator Uday Mudaliar were killed in firing by Maoists that followed a landmine blast on their convoy.
According to reports, the two had come out of their vehicles following the gunfight to surrender before the rebels but were shot dead.
The convoy of Congress party's parivartan yatra was targeted near Darbha forest in Jagdalpur, about 340 km south of Raipur.
State Congress chief Nand Kumar Patel and his newly-married son Dinesh Patel were kidnapped by the Maoists.
Senior Congress leader and former union minister V.C. Shukla was injured in the shooting and has been hospitalised.
Chhattisgarh Chief Minister Raman Singh told reporters that at least 16 people, including many policemen, were killed in the attack.
Sonia Gandhi, speaking to reporters in New Delhi after holding a late evening meeting at Prime Minister Manmohan Singh's official residence, termed the attack "despicable" and said the party was "devastated".
"Congress colleagues were taking out a parivartan yatra in Bastar and it seems the PCC president and his son, though not confirmed, it seems they may have been kidnapped," Sonia Gandhi said.
"Mahendra Karma and Udai Mudliar, we are told have been killed, and a number of them wounded. Shukla ji (V.C. Shukla) has been hit by bullets in the stomach area. We still don't know his condition."
"We just held a meeting and are getting the details. The PM spoke to the chief minister two hours ago, we don't think the CM has got back. Everyone has been told to render whatever support is possible."
Ravindra Choubey, a Congress leader from Chhattisgarh, confirmed to IANS that Mahendra Karma, the tribal face of Congress in the state, and Udai Mudliar had been killed.
Mukesh Gupta, the additional director deneral of police (intelligence) of Chhattisgarh, told reporters that a convoy of cars was passing in Sukma area when a blast hit the first or the second car.
The blast was followed by heavy firing, in which two people were injured. He said more details are awaited.
Chief Minister Raman Singh said: "This is not an attack on a particular party, but an attack on democracy."
"We must all maintain calm. I have spoken to the prime minister, and also to Rajnath Singh (BJP chief) and to Sushma Swaraj (Leader of Opposition in the Lok Sabha)," he said.
He said his government is in touch with his medical authorities in Delhi and Hyderabad.
"The fight against Maoists is long. We need to show unity," Singh said.
Bharatiya Janata Party (BJP) chief Rajnath Singh, in Gorakhpur, condemned the attack.
"I condemn it in strong words. We should fight it and rise above petty political considerations."
Former chief minister Ajit Jogi, of the Congress, accused the BJP state government of not providing adequate security.
"The state government had not provided adequate security for the yatra," Jogi said, and called upon Congress workers to observe state-wide shutdown Sunday and demand dismissal of the state government.
Congress vice president Rahul Gandhi has left for Chhattisgarh, according to sources.
Minister of State for Home R.P.N. Singh, who is slated to go to the state Sunday, has tweeted condemning the attack.
"Top priority now is finding the missing PCC chief and his son/Deepest condolences to the families of Mr Karma, Mr Mudilar & our thoughts with the politicians and securitymen battling for their lives/Sad day for democracy when representatives of the people murdered in cold blood. No cause justifies this brutality."