Indian officials have reacted with outrage to an attack by about 200 suspected Maoist rebels in which 24 people died.
The perpetrators of the attack set off a bomb and fired on a convoy carrying ruling party leaders and members in an insurgency-wracked state.
Yesterday's ambush targeted Congress party politicians returning from a campaign event with the area's indigenous tribal community.
Prime Minister Manmohan Singh and party President Sonia Gandhi visited some of the 37 people who were injured in the attack in a hospital Sunday in the Chhattisgarh state capital and said the government would take firm action against the perpetrators.
"We are devastated," said Ms Gandhi, who denounced what she called a "dastardly attack" on the country's democratic values.
Rajnath Singh, president of the opposition Hindu nationalist Bharatiya Janata Party, said the country should unite in its fight against the Maoist insurgency.
The convoy was attacked in a densely forested area about 345km south of Raipur, Chhattisgarh's capital, as the Congress members were returning from a party rally.
Four state party leaders and eight police officers were among the 24 people who were killed. Other victims were party supporters.
Police officer R.K. Vij said 11 of the 37 injured were in serious condition.
Police identified one of those dead as Mahendra Karma, a Congress party leader in Chhattisgarh who founded a local militia, the Salwa Judum, to combat the Maoist rebels.
The anti-rebel militia had to be reined in after it was accused of atrocities against tribals - indigenous people at the bottom of India's rigid social ladder.
The dead also included state Congress party chief Nand Kumar Patel and his son. The injured included former federal minister Vidya Charan Shukla, 83, police said.
The Press Trust of India news agency said the attackers blocked the road by felling trees, forcing the convoy to halt. Vij said the suspected rebels triggered a land mine that blew up one of the cars. The attackers then fired at the Congress party leaders and their supporters before fleeing.
Congress is the main opposition party in the state. It has stepped up political activities, trying to win the support of tribals, ahead of state elections scheduled to be held by December.
Prime Minister Singh has called the rebels India's biggest internal security threat. They are now present in 20 of India's 28 states and have thousands of fighters, according to the Home Ministry.
The rebels, known as Naxalites, have been fighting the central government for more than four decades, demanding land and jobs for tenant farmers and the poor. They take their name from the West Bengal village of Naxalbari where the movement began in 1967.
The fighters were inspired by Chinese Communist revolutionary leader Mao Zedong and have drawn support from displaced tribal populations opposed to corporate exploitation and official corruption.
The government has offered to begin peace talks with the rebels, but without success. The Maoists demand that it first withdraw thousands of paramilitary soldiers deployed to fight the rebels.