Heavily-armed Maoist rebels have killed at least 27 people, including Congress Party leaders, after ambushing a convoy in a remote tribal belt of central India, officials have said.
The head of the party in the state of Chhattisgarh, Nand Kumar Patel, and his son, who were kidnapped by the rebels after the attack on Saturday, have also been found dead.
"We can also confirm that 32 people are wounded, most of them seriously," state police Director General Ramniwas told AFP news agency.
A senior police officer said suspected Maoists triggered a land mine blast and fired at the vehicles in the Sukma district, about 345km south of Raipur, the capital of Chhattisgarh state.
Congress Party president Sonia Gandhi termed it a "dastardly attack" on India's democratic system.
Prime Minister Manmohan Singh said the government would take firm action against the perpetrators.
The prime minister, who visited wounded people in a Raipur hospital along with Sonia Gandhi, said that "the country would never bow down before Naxalism".
The rebels, known as Naxalites, have been fighting the federal and provincial governments for more than four decades, demanding land and jobs for tenant farmers and the poor.
Police identified one of those killed as Mahendra Karma, a Congress leader in Chhattisgarh state who founded a local militia, the Salwa Judum, to combat the Maoist rebels.
Salwa Judum had to be reined in after it was accused of atrocities against tribals - indigenous people at the bottom of India's rigid social ladder.
Congress Vice President Rahul Gandhi said his party would not be cowed down by such attacks.
"It is not an attack on Congress. It is an attack on democracy. But, we will not fear from such an attack and continue to move forward with enthusiasm," Gandhi was quoted as saying by The Hindu newspaper on its website.
The wounded Congress Party members, among them Vidya Charan Shukla, a former federal minister, were taken to a local hospital, police said.
Shukla, who is believed to have suffered bullet injuries, was later airlifted to a suburb of Delhi for further treatment.
The Press Trust of India news agency said the attackers blocked the road by felling trees and triggered a land mine blast that blew up one of the cars in the convoy. The attackers fired at the Congress Party leaders and their supporters and then fled.
The Congress Party, which runs the federal government, is the main opposition party in the state.
The rebels are inspired by Chinese Communist revolutionary leader Mao Zedong and have drawn support from displaced tribal populations opposed to corporate exploitation and official corruption.
The Indian prime minister has called them 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.
Government response 'reflexsive'
Sumit Ganguly, professor of political science at Indiana University in the US, told Al Jazeera that the government had failed to engage with its opponents.
"What is actually surprising is the government's inability to anticipate acts of this nature," he said.
"It shows the weakness of intelligence and the laxity of the government's response to this growing problem that confronts India's domestic security.
"It will probably pay attention for the next several weeks until another crisis in some other area attracts the government's attention, or distracts the government's attention.
"This is unfortunately the nature of the kind of response that this government has mounted to what is basically an endemic problem confronting India.
"Its responses have been essentially reflexive and sporadic, rather than a sustained campaign, which would address this problem on a nationwide basis."