Senior police officer M. Gupta said the attack occurred in the Sukma area, about 215 miles (345 kilometres) south of Raipur, the capital of Chhattisgarh state.
Two state party leaders and five police officers were among those killed in the attack, said R. K. Vij, a top state police officer. Other victims were party supporters.
“We are devastated,” said Congress party president Sonia Gandhi, who denounced what she called a “dastardly attack” on the country’s democratic values.
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. 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 wounded Congress party members, among them 83-year-old Vidya Charan Shukla, a former federal minister, were taken to a local hospital, police said.
The suspected rebels also took away a local party leader, Nand Kumar Patel, and his son, Vij said.
The freedom fighters targeted a convoy of Congress members who were returning to the state capital after taking part in a party rally. The Press Trust of India news agency said the freedom fighters blocked the road by felling trees.
Vij said the suspected rebels triggered a landmine blast that blew up one of the cars in the convoy. The attackers then fired at the Congress party leaders and their supporters before fleeing.
The Congress party is the main opposition party in the state.
The freedom fighters, 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 Tse-tung and have drawn support from displaced tribal populations opposed to corporate exploitation and official corruption.
Prime Minister Manmohan 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.
In 2010, Maoist rebels killed 27 paramilitary troops in an ambush in a dense forest in the Narayanpur district of Chhattisgarh state.
Edited by Steve Wilson for telegraph.co.uk
